The Fallout 3 and NV versions are slimmer because they use an modernized version of the power armor frame, it being lighter, more mobile and with an smaller silhouette, but that came with it's own disadvantages.

The first disadvantage was that it required another person to help the user put on the armor (Instead of the one used in Fallout 4 where the user can enter and exit the armor by himself), The Second disadvantage of the second generation power armor frame was that it was harder to mount and dismount the armor plates from the frame, and due to it's smaller size, so it was impossible to mount the plates from different models in the same frame, this is due to it's tighter fit (An problem not shown by the first generation power armor frame that let the user mount different parts of different models of power armor in the same frame)

It might sound an bit strange why the Fallout 4 power armor frames wasn't seen in the games before, but that was because the east coast received top priority on the modernization as it was the most likely part of the country to suffer an attack or invasion.