Three controls affect the result
Pixel dimensions determine how many pixels are stored. Format determines the encoding used. Quality controls the lossiness of supported encoders. A quality percentage is not a target file-size percentage: 80% quality does not mean a file will become 80% of its original size. Content, existing compression and browser encoding all affect the outcome.
Start with the dimensions you actually need
If a photograph is much larger than its intended display size, resize it with proportions linked before experimenting with quality. Keep the original for printing or future editing. The tool allows conversion, resizing and quality adjustment in the same operation, so you do not need to repeatedly download and re-import intermediate files.
Adjust JPG and WebP quality
Open Quality, choose Adjust quality, and start with 80% as a comparison point rather than a guaranteed optimum. The slider moves in 5% increments; you can also type an exact value. Compare a lower value with 90% and inspect fine text, gradients and edges in the preview. PNG is lossless and has no quality slider or dedicated lossless compression optimizer in this tool.
Why can the output be larger?
A source file may already be efficiently encoded, or a different format may require more bytes for its content. For supported same-format JPG and still WebP processing at unchanged dimensions, the tool keeps the original if re-encoding would make it larger. A requested resize or format change is still applied even if its output is larger. There is no guaranteed target in KB and no guarantee of zero visual loss.
What does No changes mean?
With format and dimensions unchanged, No changes preserves the original file. If you request a conversion or resize, the browser must re-encode; No changes then uses browser-default encoding quality rather than preserving the source bytes. Compression cannot restore detail lost by earlier editing. Download the prepared result and keep the source if you may need a different version later.