VERY easily! I think the people with 15's would get 2 or 3 more mpg though, but we get better handling and only 20 or 30 less miles on a tank. I still get about 500 miles on a tank, so I can't complain too much. If someone ever sells a set of 15" stock rims and tires around here I may want to just try them to see if there really is that much of a difference, but I'm not going to worry about it for now...
The Conservation of Angular Momentum is the reason you need more power to spin the additional weight of the larger metal rim. The larger rim simply requires more power because metal weighs more than rubber. So mileage is reduced and acceleration is reduced when you use a larger metal rim. It is a fact and not subject to speculation, conjecture, hunches, or other feelings of insight. This is well known to any tire or wheel professionals. So if you want more mileage use the 15 inch wheel. IF you want faster acceleration then use the 15 inch wheel. Sorry to disappoint the speculators but it is NOT the rubber compound or any other fundamental reason. You can affect mileage in a negative way with softer tires. In fact mileage improves as the tires get smoother and closer to bald. Skeptical still? Remember the skater doing a spin and she pulls her arms in and the spin accelerates and then she lets her arms out and she slows down. Or perhaps you think it is easier to lift a 20 pound weight at the end of your outstretched arm than it is to curl it close to your body where the lever arm is shorter? Smaller rims: faster and higher mileage.
I think you are mistaken in your opinion that it is only the weight of the wheel that causes the loss. Tests have shown that even if the weight of the wheel is kept constant loss will occur. The tires are almost always heavier when plus sizing so weight will go up anyway and then the rest of your post applies.
Our Canadian Touring model has the 215/45R17 (OEM Michelin Pilot). In winter I switch over to Michelin X-Ice in 195/65R15, on 15" steel Corolla rims. They seem to be about neck-and-neck for mileage, fwiw. Handling feels about the same also. Pilot's are not described as LRR anywhere, but with OEM you never know.
Summer mpg is higher than winter. 17" wheels drop mpg but 15" wheels raise mpg. 215/45/17s are smaller than 195/65/15s thus the actual miles traveled are artificially nphigh with the 17s. Do you see what is going on here?
^ Yeah, I understand that, it's about a 2% diff, revolutions per given distance. Also snows vs all-seasons, possibly a OEM LRR version of the Pilot, who knows. All I'm saying, from the last tank with the OEMs to the next tank with the snows, I don't notice much difference.