I think the operative word here is bikeS. Lots of cars, including my 46 mpg Prius, will carry one bike in the cargo space, without even taking the front wheel off. Put another bike on, and that's when you start to have trouble. And of course the smaller car, the bigger the hit it takes in gas mileage when you start hanging bikes off the outside of it.

The Prius would take two bikes one on top of the other as well, but nothing else would fit in the cargo space then.