My snakes dont move like that when they eat, but hey every snake is different and I dont have that many. As per the feeder size. That looks normal to me, they can eat much bigger than that, though I dont recomend it. I feed mine a little on the bigger size prey items, because thats all I can get.

At first is weird watching something strech out their neck, but it is fine. Im sure they dont go around in the wild hunting for their perfect size prey to eat.