On many surface mount power N-mosfets I have seen that the large pad is electrically the drain pad.
Wouldn't it make more sense to heat sink to the ground plane, where you probably already have a lot of copper? It seems like making the "heat sink pad" drain just increases the amount of total copper/used space on the board. Why design them so that you have to add an additional, or perhaps many, separate large fill(s) for heat?
