If the game takes too much space, they can just release it as a multidisc game instead of episodes that together add up to much more than a normal game. Sounds like a bad excuse at worst, but we'll have to wait and see.

Switching discs wouldn't be necessary if they make one install disc and another to keep in the console while you played. If they can't fit the game within 100 GB, the dev team is the problem, not the storage medium.