But Van Buren was cancelled and never considered canon, so it doesn't matter what it did. Even then, Caesar himself says much of the Four States are his, so his faction is indeed a major power in the west. Not sure why we're questioning the Legion's size when it is already confirmed of the territory for which they possess.
I think the mistake people are making with the Legion is that they were never meant to emulate the Roman Empire as a whole with the same culture and values, they're a slaver army and are far from what the Roman Empire was, and no doubt Caesar knows this himself. However, the main similarity is how they're formed in the concept that they absorb and assimilate the tribes they've conquered. The tribes would lose everything they stood for, their culture and identity, and become one with the Legion. In some ways similar to Romanization. For example, every tribal that was assimilated into the Legion was forced to adopt a different name by Legion customs. That's as far as the similarities go, but it's still a significant one.
Now, for the topic at hand, I don't think we'll see any of the Legion on the east coast. What reason would they have to be there? Even more so the fact that Fallout 4 takes place in 2277 (or it is assumed at least because there is nothing that suggests otherwise). By this time are the Legion at war with the NCR during the first battle of Hoover Dam. Caesar has no reason to randomly have an interest in a place that is all the way across the country when he is at war with the west coast's most powerful faction.