They made the buildings smaller iI'm guessing to try and make the game faster and less laggy. The buildings on the outside are exactly the same as the ones in SP. It's just that they are much smaller. And because they are smaller, the inside can't be as intricate as the ones in SP.
I do think however that it could be fun rushing a base and having a small assault team escourt a hotwire on the long trip down to the MCT.
But on the other hand, imagine how the elevators would affect the game.
I can imagine all kinds of fights going on over people elevator hogging, etc., etc.,...