Hi insomniac_lemon ! It's one GB because there are several thousand lines recorded in two languages, 4k backgrounds and a very complex set of music files mimicking the way iMuse works but for wave files. Everything is compressed a lot and all the character animations actually are vector graphics. Also, there are some video files in FullHD. Smaller sizes would decrease the quality considerably.
It could be an issue with the itch server. However, most people reported download times from 2-10 minutes, which we think is quite good.
Maybe you should try getting the game via the itch.io application, which should be able to resume broken downloads