JPG to WebP

The 25 to 34 per cent figure comes from encoding PNG sources into both formats and comparing at matched quality. Re-encoding an existing JPEG is a different situation. That JPEG has already thrown away the high-frequency detail WebP would also have thrown away, so a good part of the advantage was spent before you started. The saving is still real; it is usually just smaller than the headline. If the camera original or the export from your editor still exists, convert from that instead and you get the full benefit.

Both formats are lossy, so this is a second generation. One re-encode from a decent original is invisible at sensible settings. What does damage is a cycle: JPEG to WebP to JPEG and back again, each pass quantising whatever the last pass left behind. Convert once, in the direction you actually need, and hold on to the source.

There is one case where WebP is structurally the weaker format. Lossy WebP is built on VP8’s intra-frame coding, which always stores colour at half resolution in each direction and offers no full-resolution mode at all. For photographs that never shows. For fine coloured lettering on a flat background it is the format’s worst case, and an image like that belongs in PNG regardless of which lossy format you were choosing between.