Well, they're making a more single player oriented version as well, called Squadron 42. It will probably be smaller, but I doubt it'll be any smaller than these other big games (50gb). I have to try it, as I'm a Wing Commander fan and these are the creator's first games in years.
I'm not sure why Skyrim is so much smaller. I guess since it's a 2011 game, with fairly non-realistic textures, it slims it down a bit. They've upscaled the resolution in the HD pack and in SE, but not done much to enhance the detail or meshes. So maybe that's what still keeps the size down. I've also heard the audio is compressed, so that makes it smaller.