Compression is not guaranteed to reduce every file
Re-encoding means creating a new image file. If the source was already optimized, the new encoder may produce a file that is the same size or larger. This can happen even when the output looks similar.
Changing formats can also change the result. A format that is efficient for one kind of image may be a poor match for another, and a browser's encoder can produce different results from another tool's encoder.
What to try
- Try an output format suited to the image and accepted by its destination. Photos often work well in JPEG or WebP; screenshots and graphics may suit PNG or WebP.
- For JPEG or WebP, lower the quality gradually and compare details such as text, edges, and faces.
- Reduce pixel dimensions if the image will be displayed smaller. This often lowers file size more predictably than re-encoding the same dimensions.
- If the new file is larger and you do not need a different format or dimensions, keep the original.
How shrinkpixel reports the result
The tool compares the encoded output with the selected original. It reports the measured size difference and can tell you when the output is larger. It does not silently change the format to make a smaller file.
PNG has no lossy quality setting in the browser encoder used here. If PNG output is not smaller, the tool may report that it cannot reduce the file rather than claim a saving that did not happen.