Size isn't always a factor. Japan had the U.S. on the ropes for the first part of WW2, and Britain held off just about everybody everywhere for the whole war, sometimes all by themselves. Both are tiny islands compared to the U.S., Russia, and China (and Russia & China were beaten up pretty bad during that war).
I do maps, and right now I am doing a ST map, and Romulan space it tiny compared to the UFP or Klingons. In fact, I think the Klingons even have the Federation beat (we are more 'sprawling', where as they are just one contiguous mass). Cardassia is even smaller than the Romulans, and the Ferengi are a joke (aren't they always?) But my point is, it isn't about size, so much as how well you maintain your empire, and your resource distribution. The Dominion actually seemed better at that than the Borg. In fact, did the Borg produce anything other than more Borg?
It also depends on time-frame. After their little war with Species 8472, the Borg lost hundreds of cubes. That had to hurt even them. They also lost dozens of planets, but that was probably negligible. Plus, the Dominion would spend some 'prep time' (Founders = Batman!) in the Delta Quadrant making allies. The only people dumb enough to befriend the Borg is..... us. So it would be a good fight. The Dominion would be sneakier, but in the end, I think the Borg have the shear numbers. Plus, the Borg are necromancers - they get more powerful by resurrecting their fallen enemies. That means that the longer a war takes, the bigger the Borg get. They are inevitable.