[LeanBurn]

Many factors make this totally understandable.
1) How much did the 17" wheel weigh in comparison to the 15"?...17" will typically weigh a few pounds more.
2) With wheel up-sizing, tire width almost universally increases as well. 195 -> 215+.
3) OE tires on most vehicles (even on a Hybrid) are bottom rung for performance or value, moving to a better LRR tire would decrease fuel consumption.

I run 15" Prius wheels on my Corolla. At ~14lbs, they are a full 5+ lbs lighter per tire than the stock 15" steelie, the 16" or 17" wheel options... and the tire width is definitely narrower than the latter two as well. Steering response is quicker, initial steer in is snappier compared to all other options. Consequently fuel economy is better.