Why is this game's file size so large?

Does 343i just not optimize their game at all or something? MCC has 4 campaigns and a lot more content, and while that game is huge to it takes up significantly less space on my hard drive. I just want an explanation for this because it doesn’t make sense.

I mean, Halo 5 is at about 80GB now.
Really?

It really doesn’t make sense with all the fun the game is missing either.