Yeah, 11.6" can get pretty loud too, just not as loud as 13" at 6500RPM. I tried playing HL2 Ep.2 on 11" and it took it a 5-10 minutes, but the fan was going at 6500 on it. it's just that the fan is not quite as loud even when running that fast.
You can mitigate this problem significantly by installing coolbook on 13" model, lowering the voltages all the way down and then limiting the fan speed to 5000RPM using SMC Fan Control. 5000 is enough to cool it down pretty quickly after you're done with the game, and I'd say that at 5000, 13" model sounds about as loud as 11" at 6500.
As an extra precautionary step, you can remove the 1800Mhz CPU step from coolbook before running the game. I didn't see any differences in framerate, but the computer was kept cooler.