Why did my image get larger after conversion?

Conversion saves the picture in a new way, so a bigger file doesn't always mean a sharper image. A JPEG saved as PNG can keep the same flaws while taking up much more room.

FluffyBench · 7 min read · Measured October 15, 2026 with FluffyBench's PNG converter

A conversion starts with the picture, not the old file size

Think of an image file as a set of instructions for rebuilding a picture. Conversion reads those instructions, then writes a fresh set for the same picture.

The same mountain photo travels from a JPEG file through a picture in memory to a new PNG file. — New instructions can take more room without adding detail. Measurement method and sources · Photo: GlacierNPS / National Park Service; illustrations by FluffyBench, Public domain in the United States, via Wikimedia Commons.

The encoder is the part that writes the new file. It packs the picture according to the format and settings you've chosen; it doesn't aim to match the old byte count. That's why changing a file's format can make it larger even when its width and height stay the same.

Why can a PNG be much larger than a JPEG?

JPEG often saves space by smoothing or discarding tiny details. A lossless PNG keeps the pixels it's given, including any softened edges or blocky marks left by JPEG. It can't bring back detail that was removed earlier.

We saved this mountain-and-lake JPEG with FluffyBench's PNG converter, using Advanced local encoder with RGBA color. The PNG came out 5.4 times as large, with unchanged dimensions and no sharper detail in the compared patch.

The mountain and lake photo, with a small white and black rectangle marking the detail compared below. — A larger file doesn't rebuild the original scene. · Photo: GlacierNPS / National Park Service; illustrations by FluffyBench, Public domain in the United States, via Wikimedia Commons.

Look at the matching tree and rock edges: the PNG carries the JPEG's existing marks into the new copy.

Unobstructed enlarged tree and rock edges in the source JPEG. — Same detail, more bytes. The sizes are for the complete files. · Photo: GlacierNPS / National Park Service; illustrations by FluffyBench, Public domain in the United States, via Wikimedia Commons.

The same unobstructed tree and rock edges in the PNG copy. — Same detail, more bytes. The sizes are for the complete files. · Photo: GlacierNPS / National Park Service; illustrations by FluffyBench, Public domain in the United States, via Wikimedia Commons.

Why can two PNGs of the same photo differ in size?

PNG encoders can pack the same pixels in different ways. They can also store an alpha channel: an extra instruction for each pixel that says how see-through it is. RGB stores color; RGBA stores color plus that transparency instruction, even in an entirely opaque photo.

The flower's see-through surroundings need alpha. JPEG can't store alpha, so a JPG copy fills those areas with a solid color.

Side by side: a flower with see-through surroundings and a JPG copy with white fill. — The white rectangle is part of the JPG; it won't show the background behind it.

For the opaque mountain photo, keeping an alpha channel added about 300 KB with Advanced local encoder. Keeping RGBA and switching to Browser default also changed the size.

Same photo and dimensions; three ways to write its PNG

Source JPEG: 678.2 KB
Advanced local encoder: RGB (no alpha): 3.3 MB
Advanced local encoder: RGBA: 3.6 MB
Browser default: RGBA: 4.5 MB

The local pair isolates the alpha choice. The RGBA pair isolates the encoder choice.

So a size increase doesn't automatically mean you chose a higher-quality picture. It may mean you've asked for exact pixels, an extra channel, or a different way of packing the same information.

Smaller JPG, bigger PNG: which one should you keep?

Choose a format for the job you're doing. A photo for sharing, a transparent graphic and artwork that needs exact pixels have different needs.

Start with what your picture needs.

Your destination needs to accept the format you choose. A larger lossless copy can be worth the space when exact pixels matter.

A smaller file earns its place when the picture still looks right. Here are the source and a Quality 80 JPG copy, each with the same patch enlarged below.

Source mountain photo above an enlarged tree and rock detail. — Compare the whole picture at the size you'll use it and the detail you care about. Choose the smaller copy if it preserves what you need. · Photo: GlacierNPS / National Park Service; illustrations by FluffyBench, Public domain in the United States, via Wikimedia Commons.

Quality 80 JPG photo above the matching enlarged tree and rock detail. — Compare the whole picture at the size you'll use it and the detail you care about. Choose the smaller copy if it preserves what you need. · Photo: GlacierNPS / National Park Service; illustrations by FluffyBench, Public domain in the United States, via Wikimedia Commons.

For a lossy copy that can keep transparency, FluffyBench's WebP converter offers both when your destination accepts WebP.

Doing it in FluffyBench: make a smaller JPG copy

For an opaque photo, FluffyBench's JPG converter gives you a quality slider for making a smaller delivery copy.

Here's how to make one JPG alternative without changing the picture's dimensions.

Step 1. Keep your original. In FluffyBench's JPG converter, choose images; TO is already JPG.
1. Choose images opens your file picker.
2. TO shows JPG, the copy you'll make.
3. The gear button (JPG · 92%) opens Settings.
Step 2. Choose Quality 80 as a starting point and leave Size at Original.
1. Quality changes how much detail JPG keeps.
2. Original keeps the pixel dimensions.
Step 3. On a phone, close Settings with the gear button, then press Convert.
1. Convert uses the settings you chose.
Step 4. Compare the source and output sizes, then Download the copy.
1. The lower readout is the JPG copy's size.
2. Download saves the finished JPG.

Doing it in FluffyBench: a leaner PNG

If you have to keep PNG, FluffyBench's PNG converter gives you control over how a lossless copy is stored.

Here's the RGB path from choosing an opaque picture to seeing the PNG's finished size. For a picture with see-through areas, leave PNG color on RGBA.

Step 1. Choose an opaque picture; TO is already PNG. Open Settings with the gear button.
1. Choose images opens your file picker.
2. TO shows PNG, the copy you’ll make.
3. The gear button (PNG) opens Settings.
Step 2. More options reveals PNG encoding. Advanced local encoder lets you choose the color channels.
1. Open More options.
2. Choose Advanced local encoder.
Step 3. For an opaque picture, choose RGB (no alpha) to leave out the transparency channel.
1. Choose RGB (no alpha).
Step 4. On a phone, close Settings with the gear button, then press Convert 1 to PNG.
1. Convert makes the RGB PNG.
Step 5. The RGB PNG is ready. Compare its lower size readout with the source above it.
1. The upper readout is the source JPEG’s size.
2. The lower readout is the RGB PNG’s size.

How we measured

Date: October 15, 2026. Browser: Chrome 153.0.8010.12. Device: AMD Ryzen 9 5950X, Windows 11 Pro.

FluffyBench's PNG converter used its custom encoder at compression level 6 and 8-bit color, once with RGB and once with RGBA, plus its browser-encoder default. Resizing was off for this run. PNG has no lossy quality control; the tool's Size controls can still resize it.

One 1,920 x 1,276 JPEG of 678,202 bytes (Stanton Mountain reflected in Lake McDonald).

File sizes are exact bytes; KB means 1,000 bytes. Uncompressed RGB samples would occupy 7,349,760 bytes (width x height x three channels); that is pixel data, not a saved image file.

The recorded PNG outputs are 3,332,330 bytes with RGB, 3,640,163 bytes with RGBA and 4,527,150 bytes with the browser encoder's RGBA output. Both local outputs used 8-bit samples and compression level 6 (labelled Balanced in the current UI).

The original decoded-pixel comparison found 0 changed positions in a 128 x 96 crop (12,288 positions). The new display crops use that region without the guide grid. The PNG was recreated with the current local encoder and its byte count checked against the recorded output.

The JPG walkthrough and final comparison are rendered for illustration with FluffyBench's Standard encoder at Quality 80 and Size Original. The final image shows the whole photo and a matching enlarged detail; its file size comes from the rendered JPG, not the earlier PNG measurement.

Format behavior was checked against the linked W3C PNG specification, ITU-T T.81 JPEG recommendation, Google's WebP documentation and MDN's image-format guide. The flower is a visual explanation of transparency, with no size-saving claim.

Image credits

Stanton Mountain reflected in Lake McDonald, conversion illustration — GlacierNPS / National Park Service; illustrations by FluffyBench. Public domain in the United States. Recreated PNG at the recorded encoder settings, clean matched crops, a conversion diagram, or a JPG workflow rendered for illustration.