PRACTICAL GUIDE
Why Did My Compressed Image Come Back Bigger?
Compression tools sometimes return a file larger than the one you gave them. Here is why it happens, why PNG is the usual culprit, and what to do instead.
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Compressing and re-encoding are not the same thing
Almost every online image compressor works by decoding your picture into raw pixels and then saving it again with its own encoder settings. The result is a brand-new file. If that encoder happens to be less efficient than the one that produced your original, the new file will be larger — even though nothing about the image got better or worse.
So a bigger output is not a sign that your image is broken. It means the second encoder did a worse job than the first one, which is more common than most tools admit.
PNG is where this happens most often
PNG is a lossless format, so there is no quality dial to turn. A quality setting of 60% and one of 95% produce exactly the same PNG. Anything that claims otherwise is quietly converting your file to something else.
PNGs also carry optimisations that a re-encode throws away. A graphic exported by a design tool may use an indexed palette at 8 bits per pixel, or a reduced bit depth, or a carefully chosen filter strategy. When a browser canvas re-saves it, it writes plain 32-bit RGBA and lets the generic compressor deal with it. A 400 KB palette PNG can easily come back at 1.8 MB.
JPEGs can grow too
If a photo was originally saved at around 70% quality and you re-save it at 90%, the encoder spends more bits describing compression artefacts that are now part of the picture. You get a larger file that looks no better — and slightly worse, because every JPEG save discards a little more detail. This is called generation loss, and it accumulates every time the file makes another round trip.
A compressor should refuse to hand you a bigger file
This is the part many tools get wrong. They re-encode, see a larger result, and hand it over labelled 'compressed' anyway. The user ends up with a heavier file and no warning.
Our compressor compares the two before deciding. If the re-encoded version is not smaller than what you supplied, it gives you back your original file untouched and says 'already optimised, original kept'. That message is not a failure — it is the tool telling you the truth about your image.
What to try when the file really is already optimised
Reduce the dimensions. Pixel count is usually the dominant factor in file size, and it is the lever compression cannot pull for you. Halving both the width and the height removes about three-quarters of the pixels.
Change format. A PNG photograph is often several times larger than the same picture as a JPG or WebP. A PNG that contains a photo rather than flat graphics is almost always in the wrong format.
Accept it. A well-exported 180 KB JPEG at the size you actually display it has nothing left to give, and squeezing it further only degrades it.
Frequently asked questions
Does 'already optimised, original kept' mean the tool failed?
No. It means the tool tried to compress your file, found that the result would be larger than the original, and returned the original instead. Handing back a bigger file and calling it compressed would be the failure.
Why does the quality slider do nothing to my PNG?
PNG compression is lossless, so there is no quality parameter to apply. In browsers, the quality argument used when saving an image is honoured for JPEG and WebP and ignored for PNG.
Will converting to WebP always produce a smaller file?
Usually, but not always. WebP is very efficient for photographs and for graphics with large flat areas, and it also supports transparency. For small icons or already heavily optimised PNGs, the difference can be marginal or even negative.
Is it safe to compress the same image twice?
For PNG, yes, because it is lossless. For JPEG and WebP, each lossy save throws away a little more detail, so it is better to go back to the original and compress once with the setting you want.