Well, the way I'm rationalizing it to make it sound less lame is if hundreds of Hollow pile on top of each other, you get a Gillian (so hundreds of souls). Eventually some developed intellect and to get stronger ate other Hollows (so add on all the souls they ate). And then the ones that had become the strongest, thus eaten the most, eventually became Aizen's espada. Since they only moved the people, couldn't move the actual land of the city, it kinda makes sense... right?
Of course.. I could 've just put more thought into it than Kubo, and he's just gonna go with "these are super strong souls, they equal 100,000 so it all works out!!!"