As you say.
Just as an example, these 15" rims are ~8lbs, some of the lightest you can get in that size:
The Insight's stock 14's are 11lbs on my scale, 3lbs heavier than the above rim:
However, the "dish" of the larger rim is a half inch farther out, and it would appear that most if the weight savings have come from reduction of mass in the spokes and center, so it may well be that, with a tire that gives the same revs/mile, the heavier rim could have lower rotational inertia due to the way it's distributed.
I could see that applying to steel as well, but I'm not finding any steel wheels in my size that are even remotely close to 11lbs plus the weight of moons.