Class and Level Geekery VIII (spoilers ahoy!)

[zimmerwald1915]

And "a creature can carry as many passengers as its space and carrying capacity allow. One Small or Medium sized rider or passenger can fit in each 5 ft square the creature occupies and still have room to move around and fight if necessary"

Which is just the point, really. A large creature has four spaces a passenger could occupy. However, we see the slavers fleeing with captured slaves - this would put a large creature over its four-passenger limit. Hence, huge creature.