Well, the first thing to remember is that the division wasn't arbitrary. The territory of the Caesar of the West, for example, corresponds to the breakaway Gallic Empire of the late third century. Which is not to say that Diocletian was respecting the borders of the Gallic Empire, but rather that the forces leading it to take the shape it did were the same lead Diocletian to draw the borders that he did. It is also important to remember that the split wasn't supposed to be absolute--the various leaders were still supposed to help each other out, just that now they had smaller territories to worry about.

But I think more importantly, this was not a division of resources but a division of responsibility, particularly in regards to the Rhine-Danube frontier. So if you look at a map of the tetrarchic divisions you will see that there is a qualitative division between the Augusti and the Caesari. The Augusti have the economic heartlands (Italy, Spain and North Africa in the West, Asia, Syria and Egypt in the East) while the Caesari have the frontiers (Rhine for the West, Danube for the East). The Eastern Augustus did have the Persian frontier, but that was a very different thing than the Rhine-Danube.