Compression, most likely.

Yes, compression. Same with WebP. I see so many RenPy games that use webp. Curiously i have a script that scans images (since i get so many errors with my optimizing programs) that checks if the files are what they say they are, and i'll find all the time .jpg's are really png or webp, or png's are jpeg or webp.

Though trying to recompress webm files is VEEEERY slow, unless they find some speedups with ffmpeg later. Also seems you can only really do 1 core per file/stream vs having most of your cores work on a video like i see ffmpeg enjoy doing.

Still for recompressing VN's i did notice doing a quality of 90 (with webP) is nearly imperceptibly lossless and good enough for making a lot of these games smaller without any big issues. Though a number of VN's i see coming out are also using CGI so everything looks like it's from a game or blender and really doesn't look/feel right.