The alternative would have been quadruple the cell requirement for interior cells. That, or removing the possibility for multi-level buildings without loading. Granted, yes, this game is certainly optimized for outdoor environments, the trade off here for true Z axis freedom is HUGE. When working with an engine, I would never - ever - ever, give up z access freedom for quadruple interior tile requirements or constant loading so you could have better interior maps.
Actually, I would rather have the interior mapping working good than complete Z freedom
INSIDE BUILDINGS. There are very few spots in existing game where this is even an issue: the large, multistory factory type rooms, and the rooms with holes in the floor to drop to floor below, some "atrium" type areas, and going down a staircase. But many staircases are already a demarcation point of a new area/cell and you have a short (1 sec) load as you open a door at the top or bottom of stairs. The large multistory could be a problem;'ll have to think about that some more, but it would not be a big loss to lose those rooms. The "atrium" areas would be the biggest issue - they do add play value/realism, shooting down (or up) at enemies on the other floor.
A little thought in planning could likely solve the atrium problem: have the room be empty of "mappable obstructions" (i.e., anything that would show up on the map), then that cell could be add to the Pipboy "current layer only" map on both applicable floors and would show as a large empty area; by preference, always put a stairway in the atrium area for easy check out of door placement - which would be unclear from the Pipboy map ( which IS the existing problem anyway)
With smaller interior cells, there's no reason not to load the current cell as well as
all adjacent interior cells from HDD - should be possible to use only current cell in processing while keeping adjacent cells almost instantly available in RAM. Keep track of which cells are already loaded,and keep loading newly adjacent cells in the background if possible (should be, since they'd be smaller)
4X number of cells yes,but very little more information contained, should be almost the same size in memory, simpler to develop and debug it would seem to me