I'd say that the difference is that in space opera there are an infinife number of spaceships around. So in every book Honor Harrington goes from being in charge of 10^n spaceships to being in charge of 10^(n+1) spaceships, and she never runs out of spaceships. Fantasy novels tend to be set on small parts of individual worlds with finite numbers of orcs, unicorns, and gods. I'd say the limit to the fantasy power curve has a lot more to do with the limited size of the world rather than whether the source of power is the characters' magical powers, or their gadgets, or the armies under their command.