Sorry I haven't gotten back here sooner, Ark-recurve.
Here's what I have noticed. The larger animals are no where near as jumpy. Doing pure guess work, I'd say it's a combination of two things: animals with larger bodies can't move quite as fast as wee animals, and they aren't quite as nervous about predators, because there are fewer predators big enough to kill them.
Impala have to worry about EVERY meat eater, from Python, to Jackal, to Leopard. A Kudu, Gemsbok, or Waterbuck only has to worry about Lion and possibly Leopard.
That's my take on it.
Too Short