This isn't a heat issue, but a cooling issue. The issue is how well that heat can be dissipated and keep the cpu cool. Smaller die size means less surface area used to draw heat away from the cpu. They are just harder to cool.
Larger heat sinks, with more airflow, and liquid cooling have become more of a necessity with Ivy Bridge cpu's - especially when overclocking.
the limiting factor is the link between the chip and the heat spreader, any changes made beyond that point in the thermal chain will be of minimal effectiveness. Yes keeping the heat spreader side of the junction helps, but no where near as much as it would if that junction was a better thermal conductor.