AC does affect performance and fuel economy.

 

You guys from out west just don't understand the emotional attachment we here in the south have for A/C*.

As for the size of the cabin, the engineers take this into consideration when sizing the A/C. In the Fit's case it's merely adequate, but this means it takes too long to cool down after sitting in the sun for a few minutes. If it's parked outside and your drive to work is less than 10 minutes you're going to be sweating.

*I've bought cars in Seattle and Redwood City. A/C function wasn't much of an issue, however I drove those 2 same cars to Atlanta in 1996 and they performed very well in GA. Mercury Tracer (wagon) POS, but at least its Ford/Mazda A/C did OK. Ditto for the Nissan Quest (minivan) with it's Ford A/C. The '97 Honda Odyssey (replaced the Quest) did much better than the Fit although not as well as the Quest.