It's true because if more vault space was added people on last gen would get really mad. It also causes downgrade in graphics, let me explain: developers have a limited amount of time and people to create a game, they can split there resources and time to four consoles or just two. If they focused on just two consoles they have more resources, more people on each team to make the game better, or focus on all 4 consoles and have less resources for each team, and less people per team.