DK64 required the expansion pak due to a bug (it was intended to work in either 4MB or 8MB modes and they couldn't fix a crashing bug in the 4MB version) and then Rare spent an obscene amount of money shipping an expansion pak with every copy of the game. That was probably responsible for a large fraction of the units shipped, with most of the rest accounted for by Zelda (which did not include the pak even though it was required).