base plate
Seriously now,
I haven't really studied it, but two things come to mind, size and thickness. Thickness is important to spread the heat quickly to the outlying fins. Therefore, I think that the larger the sink, the thicker the baseplate should be. But I have no quick numbers.
As for size, obviously if you need more cooling cap, you need more real estate to radiate and/or conduct the heat to the ambient air. There is a limit to the number of fins you can put on an inch and the size of the fins with respect to their distance, because if you put them too close they will heat each other, unless you use forced air. So, closely-spaced fins need forced air to be effective. With natural convection you need to get larger area with higher capacity. But this is all common sense.
Bottom line, as I see it, thicker baseplate will make the sink more effective all other things being equal.
Does this help you?
Jan Didden