JPEG and WebP quality values are encoder inputs, not percentages of retained detail. A quality of 80 does not mean eighty percent visual fidelity or eighty percent of the source bytes. Content, dimensions, browser codec, alpha and source encoding all influence the result, so inspect output rather than treating the slider as a universal scale.
Compare at the final display size
Inspect the preview at the scale where the image will actually appear, then zoom into text, sharp edges and gradients. Compression artifacts can hide in a thumbnail and become obvious later. ImageCompress keeps dimensions fixed, so any byte reduction comes from the selected lossy encoding rather than fewer pixels.
Know what re-encoding changes
ImageCompress preserves decoded pixel dimensions, not the original encoding. It does not promise metadata, ICC profiles, DPI, animation, byte identity, losslessness or professional colour fidelity. JPEG replaces transparency with the selected matte. Use ImageResize separately when changing dimensions is acceptable and a smaller byte budget cannot be reached by quality alone.
Inspect the actual output
The preview and download point to the same browser-created Blob, so compare edges, gradients, text and transparent areas before using it. The byte count is exact for that Blob, but another browser or codec version may produce different bytes from the same visible settings. Keep the original file until the destination accepts the result.