If you mixed lossy and lossless, I might have found the lossless ones ;-)
I had downloaded the game, went to some folders and picked a file. It was 1MB and was a background. The next picture was the same background with a character. I assumed those files to be lossless webp. They were around 100kB after I recompressed them.
For reference, I am now using the first screenshot. It is png with odd resolution of 794x446. It compresses as lossless in webp as 353kB and with lossy setting 90 down to 49kB. The default 75 is only 18 kB.
This should get more efficient with higher resolution. Meaning 90 is a bit high. Your output is that of a rendering software. That should compress very good in lower lossy settings like 75. Maybe some of the presets can help there too, but I am not sure which one. 90 was needed for that odd resolution pic. The strap of the bra looks blurry, with lower setting. The png does not look good overall, and I can't pinpoint exactly why. Maybe because of the low resolution.
If you reuse backgrounds a lot, you can do so in renpy and use transparancy features.
Anyway, a factor of 20 for png masters and 10 for webp masters is expected in shrinkage by using lossy webp. And I have seen far too many renpy games that use png and complain about size limits. At least you used webp, but I assumed it is lossless because of my small sample size ;-)