When I open an item in my Library to install, itch app wait quite a while before it can determine the install list (game, extra) and required disk space.

Then I realize butler is downloading a several MB data to determine such information. Why? Is the metadata json (or whatever format) that big? Seems like this could be optimized to provide a better user experience.

It actually determines the install size by downloading part of the archive so it can look into it and see how much space it will take up if I remember correctly. There is no metadata we store on the server about the size of these archives.