Bay's claim to no mass displacement was immediately debunked by the Cube scene in the first movie. But let's assume mass displacement didn't exist. Looking at each Cybertronian and their alt modes, can you say that anyone of them would have space in their vehicle modes for humans especially after the introduction of stealth modes? I can only see Starscream and Jetfire because the space for room is on top of and in a small area of their bodies. Perhaps Ironhide and maybe a couple of those random construction bots.