With SSD they don't need to repeat some data many times as they did with HDD, and now post launch patches will be smaller, and they won't need normals, LoD, etc, and now they use a better compression. A 50GB of the current gen game but made for PS5 would take way less space of HDD, probably even like way less than the half. In addition to this, I'm pretty sure they'll have some smart way to optimize SSD data usage as mentioned by Cerny I think it was at Wired, where you only install the part you'll use (let's say core of the game + episode 3 of the single player mode that you are playing, and maybe episode 4 and once it's completed episode 3 is deleted until you want to play it again).
They won't need 200GB, because with 100GB they will be able to store there way more than they did in 100GB in the last gen, and because the PS5 blurays will be 100GB. And that will be more than enough for most games.