Stop saying that the other dimension is vast! it could not be vast!
Tobi appears to be the fastes character in narutoverse, traveling at the speed of light according to Zetsu. Which means that if all Tobi can use is one jutso all Tobi is making is to enter the other dimension and traveling there. If said dimension was huge it would take similar time to go through it than it would to travel in the real one. As this is not the case we must assume that Tobi's dimension is far smaller than ours. In fact so incredibly smaller that when travelling great distances it would seem that Tobi would get there instantaneously.