How target-size compression works
File size depends on the image itself — a detailed landscape needs far more data than a flat graphic at the same quality. So instead of guessing, the tool encodes the image repeatedly and searches for the highest quality that still fits your target.
If a target is too small for the image's dimensions, lowering quality further would make it blotchy. With “Allow smaller dimensions” on, the tool reduces the pixel size just enough to fit and tells you the new dimensions. With it off, you get the closest result and a clear explanation instead of a ruined image.
Which format to choose
“Keep format” re-compresses in the original format. Converting can save much more:
- JPG — best for photos; smallest files with universal support
- WebP — typically 25–35% smaller than JPG at similar quality, for websites and modern apps
- PNG — keeps transparency; compressed by reducing the colour palette (great for screenshots and graphics, not photos)
When compression won't help
Images that were already compressed hard may not get smaller by re-encoding. If a result isn't smaller than your original, the tool says so rather than handing you a bigger file. Resizing the image is usually the most effective next step.