Methinks I smell the scent of a controversial topic here!
I don't know whether the head size increase by eating larger prey is valid (possibly not, I suspect), but it's an interesting theory nonetheless.
Wouldn't it be possible that the stretching/exercising involved in eating large prey mean they would develop larger jaw muscles plus widening the gap between the jaw bones? Admittedly this would not impact on the size of the top part of the skull... thus resulting in a big-jawed, small craniumed monstrosity!

Hmmm... the more I type the more I'm convincing myself that you're right, Nicole!

Better stop!
I'd be VERY interested if Steve or Fangs could get a fuller explanation from the breeder as to why exactly they think this is valid....