PNG to JPG

Why the same picture is so much smaller as a JPG

The two formats compress in fundamentally different ways. PNG is lossless: it looks for exactly repeating patterns and stores them compactly, but it must be able to reconstruct every original pixel perfectly. In a photograph, almost no pixel exactly repeats — a clear sky is thousands of subtly different blues — so PNG finds very little to exploit and the file stays huge. JPG is lossy: it works in terms of visual frequency, keeping the broad shapes your eye latches onto and quietly discarding the fine variations it will not miss. That is a superb strategy for a photo and a poor one for a diagram, which is why photographs shrink so spectacularly and flat graphics sometimes do not.

The transparency trap

This is the single most common surprise in PNG to JPG conversion. JPG has no alpha channel — the format was designed for photographs, and photographs are opaque rectangles. When a PNG with transparent regions is converted, those regions must become some actual colour, and the near-universal convention is white. So the logo that floated cleanly on your website suddenly arrives with a white box around it, and on a dark slide that box is glaring. There is no setting that fixes this, because the limitation is in the format itself. If you need both transparency and a small file, WebP is the format that gives you both.