The fans on my 15" spin up more often but at the same time, I have yet to actually feel my 15" MBO get hot. The fans do a very good job at keeping it cool to the touch. Even after an hour of converting some videos, the case barely felt warm.
There are two fans on the 15" and one one the 13". The fans on the 15" continuously spin, when cooled at about 2000RPM or so, and on the 13", they frequently turn off.