Small skeletals with better performance than bigger skeletals?

Hey there,
I have a little problem…

Till 20 fps down, I can spawn about 300 chickens.
The same 20 fps limit is arrived with only 50 elephants.

Same problem if I scale the chicken up to the elephants size.

The graphics-settings make no difference, its always the same performance no matter if low or epic settings.

The Performance depends completely on CPU, but I don´t understand why a big skeletal needs more performance than a small one…

Would be nice if anyone could help me ^^