Why Is a Compressed File Larger Than the Original?
If a file ends up larger after compression, the tool flags it clearly as "grew" in the results — this isn't a bug, and it's almost always caused by an output format that doesn't fit the image's actual content.
Why this happens
PNG is a lossless format designed for icons, screenshots, and flat-color content with a limited color range, and it's efficient there. But encode a photo with rich color gradients as PNG, and lossless compression can't shed detail the way a lossy format can — the result is often much larger than an efficient lossy format (JPG, WebP), sometimes even larger than the original.
| Original format | Wrong output choice | Result |
|---|---|---|
| A JPG photo | PNG | Can grow noticeably |
| An already heavily compressed JPG | Re-encoded again at a higher JPG quality | May stay about the same or grow slightly |
How to fix it
Choose JPG or WebP for photographic content — people, landscapes, product photos. PNG is really only suited to screenshots, icons, and anything needing a transparent background. For a mixed batch, "auto" avoids forcing every image into the same possibly-wrong format if you're unsure.
Your files are processed in the current browser and are not uploaded as article samples.