That size is because the game is essentially screenshots made from Koikatsu Studio.
A lot of VNs with characters made with Koikatsu characters do it the same way, that's why their sizes is so big.
For example, the first pajamas party with Jaiden contains 283 pictures and video with them mostly being pictures.
View attachment 1356836
That event alone is 74MB in size and that's the heavily compressed version!
It's very different from most other games as they use models in game and only save those animation. Those take up way less space as the only thing different is the position and the orientation of the models.
This is also why you probably will not see our characters wear different clothing in the same event. He has to redo the whole event, taking new screenshots with the other clothing. It's a lot of hardwork, but it is one of only ways that is accessible for indie devs without a lot of game making knowledge.