About I always just assumed that the volume or "size" of the universe was constant, not expanding. It's simply that all its constituent pieces are shrinking. Space isn't expanding, everything is just getting farther apart because the volume of every particle in the universe is shrinking at an ever increasing rate, thus giving the appearance that any two objects in the universe are getting farther apart.
We just can't notice this affect because as everything shrinks at the same rate in relation to every other particle subjective or relative size appears to be stable. It is only when compared to the stable volume of the universe that this effect is even noticeable.