This is true, but consider that I was not comparing apples to apples with a 225/60/15 vs 275's and 295's.
For example, a 295/50/15 BFG T/A tire weighs in at 32 lbs. Even if you assumed the 15x9 or 10 that you'd need to mount it weighed the same 25 lbs as the 15x7, that would make for a 57 lb wheel. That's almost 8 lbs heavier than the 295/35/18, and I would guess that the moment of inertia would be higher as well.
Sidewalls are not a constant thickness, they taper and carry a lot more weight as they move away from the rim. To truly figure out the moment of inertia you'd need the weight distribution of the tire.
Of course, not all tires are as heavy as the BFG T/A's either. Still, I don't think that the moment of inertia discussion is as clear cut as it might seem on the surface.