Rebirth is almost 150GB. In Revelation, they’re cutting the Mythril Mines and some places like the Gongaga reactor that you wouldn’t return to at this point of the game, and they’re adding the Northern Crater, Wutai, Mideel, Rocket Town (remade as a space center), and Midgar is back. They’re not adding full underwater exploration, but there are underwater events.

I don’t know how this game can be under 250GB.