What is a Fitgirl repack?
So when you have a game's install files, you have many different elements, which everyone may not need.
You'll have your textures, your sound files, your models, and so on.
Now, if I'm downloading a game, I may not need the 4k textures, because I can only run it up to 1080p.
Or I won't need the Spanish and German language packs.
Fitgirl separates all of these into optional pieces and uses compression algorithms to reduce the size further.
You get the same game, but you can cut the fluff and slim it a bit to make for a smaller download.
You pick and choose the relevant pieces, and ignore what you don't want or need.