The engineers that design the notebook custom design the heatsink to fit in the space alloted. They may use any number of fan sizes and then they test their creations with temperature probes to see if they were right.
Some of retail processors come with fan and some do not. For notebooks with processors that do not come with fan, what kind of cooling system is used? Do they use separate fan? Wouldn't other type of cooling system be preferable, if there is any, because of quietness of operation?