Do they really need an animal companion that is really powerful?
Rangers on the other hand are either archers and thus need something to keep foes off of them or to ride around and shoot from. Or are two-weapon fighters that need someone to flank with or assist with to counterbalance the penalties to TWF. It would seem from a power perspective, it would make more sense for the Ranger to have the “full” animal companion and the druid to have the “half” animal companion.