It's not necessary now, but the author just said they were going to do monthly updates. At 1 gigabyte per month, the game will eventually become harder to host/download. Many games that are ten times longer are this size. Probably because the images are unnecessarily large. I got the tips on what programs to use from devs in other threads. Obviously I'm not forcing anyone to do anything, I was merely bringing it up :-/
edit:
Just saw the author reply, well, if you're already doing it, that's great! No need to do it any harder. Images like these typically have a big savings, but if you're already taking advantage of that savings, well then, there's nothing more you can do.
Actually I see you're already using webp, which is what these tools do best, I wonder though, am I crazy? wasn't an older version of the game .png's? or am I confused? (did you start converting the file between version 0.01 and 0.02?) Or did I just misremember?