Its pretty straight forward, the tires are significantly bigger, that causes two things, overall size is greater now(i know it looks better but it affects economy) and you have more un-sprung mass. Also the tire rubber compound on stock hybrid tires is always engineered for a low co-eff of friction and better economy. The last factor we cant really fight but the other two you can, I know because have experienced a 2009 prius for 3 years, changing tires and rims brought down average consistently by around 2-2.5 km/l as compared to what was achieved before(was still around 14). It seems trifling but it actually makes that much of a difference, you can see that's why Indus motor company switched to thinner tires when they upped the radius of the rims so that overall diameter remains constant.
