A dumb AMD question

This has probably been asked a dozen times but I figured I'll ask again. Why is the die so small on most AMD cpu's? If they made the surface area larger wouldn't that promote much better cooling? My Athlon XP runs ok temp wise but I wonder how much better it would be with 3 time the surface area?

Then wouldn't the cost of making the CPU be 3 times as much?

I guess that would be hahaha.