Drop JPGs. Each is written as WebP, with the WebP quality searched until the result measures visually identical to your JPG. The byte saving is the measured difference between the two files, not an average.
The saving, measured
Articles quote WebP as about a quarter smaller than JPEG. That is an average over someone else's pictures. Here the WebP is actually written, searched to the same measured quality as your JPG, and its real byte count is shown beside the original's.
Sometimes the answer is that there is no saving: a JPEG from a good encoder can already be close to what WebP manages at the same quality. The page says so rather than handing you a larger file. Switch the target to Best per picture and the converter recommends keeping your original in that case.