What Causes rMBP Fan Noise When Connected to Two External Monitors
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.