Holy Cats.
I have never seen a Barnes give that kind of entry before. I would expect an exit about 1/2 that size.
A 100 gr in the 257 Weatherby can be REALLY fast. Over 3500 FPS Muzzle Velocity is not out of the question.
The splash effect of that bullet hitting a rib must have caused that large hole. That would be my best guess.
How far away was that deer?
Did you get an exit?