(This is a very old version of Mutant Standard, but it's still illustrative.)
I think Lossy WebP is a bad format, mainly because the generation loss is really bad and it doesn't appreciably provide any feature improvements. (I'm all on the JPEG XL train because it's so much more holistic as a format and actually does some really handy things for compression and web image delivery that wouldn't just be good for big CDNs or whatever.)
So, Mutant Standard has more than 8000 emoji in the current set because of all the modifiers, that's a lot of images, and it can be slow to load them in say, an emoji picker. I've played with image formats (including experimental/next gen ones) a lot because for me, the more accessible Mutant Standard is, the better.
In my own tests, Lossless WebPs are appreciably smaller for Mutant Standard emoji at least than compared to a crushed PNG, that's helpful, especially for people in the US and elsewhere with poor internet connectivity.