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There seems to be some confusion going on. First, afik, aluminum wheels of a decent quality, will almost always be lighter than steel wheels of the same size.
Next, larger diameter wheels do not necessarily imply a larger rolling diameter! They will however, be heavier than wheels of a smaller diameter and, more importantly, have a larger rotational moment of inertia. For example, my stock RX-8 wheel diameter is 18" and tire size is 225/45x18 which gives 800 rotations/mile. If I wished to improve mpg's by reducing rpm at a given speed, I could use a 235/50x18 to get 760 revs/mile. Or, keeping the stock overall gearing, I could go to a 17" wheel and use a 235/50x17 or 215/55x17 and get a lighter wheel assembly. One could go even further with a 225/55x16 tire/wheel assembly and still be pretty close to stock gearing.
For mpgs, one is overall much better off with a small diameter wheel and a "tall" tire. The rim is the densest component, so moving that inwards (even with the same weight) reduces the moment of inertia. Additional advantages include smoother ride and increased resistance to blow-outs from rough roads. Also, tires with taller sidewalls are easier to make and generally cheaper. Disadvantages include reduced maximum cornering limits and "sloppy" handling.
All the data is available for each tire model on Tirerack.com under the "specs" headers.
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