It wouldn't surprise me if the i3/i5's did use less power at idle. And maybe even under some CPU only loads as well. I'm sure they've been optimized for good power conservation.
I can also see a heavy 3D graphics session pushing the power requirements above what the CPU cores alone would use.
That max thermal design power rating isn't the best guide to use talking about 'heating' and cooling requirements but it's the only one Intel provides.
More transistors can mean more heating. But E8500 with 410 million transistors @ 45nm also has a Max TDP of 65watts.