They produce 100% more heat.
They have 100% more die area, and a 100% higher TDP
Core 2 Duo e6700 2.66GHz*2 : 65W TDP
Core 2 Extreme QX6700 2.66GHz*4 : 130W TDP.
This does not mean that the CPU will go from 50°C to 100°C, apart from anything else you are doubling the number from an arbitrary zero point, wouldn't it make more sense to start from 0°K ?
However, 100% increased heat output does not equal 100% increased temps. It is harder to cool a quad core CPU, but with a decent air cooling solution Core 2 Duo is limited more by the maximum voltage you want to put through it than temps in my experience. With water, the higher TDP would not be an issue.