I believe the original raids were 20-man or 40-man, as the size options.

In BC, they reduced that to 10-man's and 25-man's.
And have stuck with that through WotLK, and stated they're happy with that balance size for Cataclysm too.

I agree, having the larger raid as an whole number multiple of the small raid would make sense.
Whether its 2x10 or 3x10, at least the entirety of your guild could do both 10's and the larger run.