How image compression works
JPG and WebP are lossy formats: they throw away detail your eye is unlikely to notice — subtle colour shifts in smooth areas, fine texture in busy ones — and store what's left very efficiently. The quality slider controls how much is thrown away. PNG is lossless: it never discards detail, so its quality setting does nothing, and the only way to make a PNG much smaller is to reduce its dimensions or convert it to a lossy format.
The biggest saving usually isn't the quality setting at all — it's the size. A photo straight off a phone is often 4000 pixels wide. Displayed on a website at 1200 pixels, three-quarters of those pixels are downloaded and then thrown away by the browser. Capping the longest side at 1920 or 1280 px often cuts the file by 80% before quality even comes into it.
JPG vs PNG vs WebP
Use WebP for anything going on a website: it's typically 25–35% smaller than a JPG of the same visual quality and it supports transparency. Use JPG when you need the widest compatibility — email attachments, older software, print shops. Use PNG for screenshots, logos, diagrams and graphics with large flat areas and sharp text, where lossy compression would leave smudges around the edges.
If a compressed file comes out larger than the original, the original was already well optimised (or you've converted a small graphic to a less suitable format). The tool marks those files so you can simply keep the original.
Why smaller images matter
Images are usually the heaviest part of a web page. Smaller images load faster, use less of your visitors' mobile data, and improve Core Web Vitals — the page-speed signals Google uses in search ranking. For email, smaller attachments avoid size limits; for WhatsApp and social uploads, starting smaller means less re-compression by the platform.