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If you're comparing to the mid-2009 base 15", I don't see why one would run hotter than the other, both have the same hardware. I can only see the 15" running hotter than the 13" if and when it's running on the 9600M GT, other than that temperatures should be relatively the same.
 
When using the 9400M, the 15" will run quieter, but not more cool. The dual fans in the 15" can keep the 9400M cool during gaming with the fans spinning at ~4000 rpm. It's pretty quiet.

The MBP 13" rev up the fan to 6200 rpm when gaming.

Basically the temperature is the same but the dual fan system in the 15" (not the current low-end 15) will keep the machine quiet while keeping it cool.

I hope that made any sense. :)
 
Just wondering.

With most usages, the 15" will run cooler than the 13", I have used the 13" uMBP and have a new 15"uMBP, the 13 always got uncomfortably hot, whereas my 15, using the 9600 for Final Cut, was cooler. It does have two fans, but it also has more airflow as it is a bigger machine, and the parts are not as tightly packed, so not only is mine quiet as a whisper, it is cool to the touch as well
 
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