What Causes rMBP Fan Noise When Connected to Two External Monitors Connecting two external monitors to my Retina Macbook Pro 2012 has no problem except for more fan noise, which is a little bit annoying since with one external monitor one doesn't hear the noise at all. 

I'm wondering what's the cause of this? What is the bottleneck here? Is it because GPU Clock Speed or Memory of Graphics Card?

rMBP 2012 has Nvidia GeForce GT 650M with 1 GB GDDR5. If I switch to rMBP 2013/2014 which has Nvidia GeForce GT 750M with 2 GB GDDR5 and say if the problem disappears (I'm just guessing), is it mainly because of higher GPU speed or more memory? 

Btw, 650M scores 1278 while 750M scores 1461.

Update 01/25/15: iStat Menu: 
iStat Menu monitors both CPU/GPU usage and temperature. Compared to only one external monitor, two external monitors causes CPU &amp; GPU temperature up by 20F higher. However, CPU &amp; GPU usage &amp; GPU memory usage all remain the same. 

The bottom line is that iStat Menu can't explain the increase in CPU/GPU temperature.
I would recommend the iStat Monitor to find out what is going on.

In comparison the 750 vs 650:

50/45 watt power consumption

and 967 vs 900 MHz clock 

would not explain the higher heat generation.

However due to the RAM size difference it could explain that the 650 has to work harder (at max power) resulting in higher heat generation, while the 750 does not break the sweat with 2 monitors.