At the First Joint Congress for Evolutionary Biology in Ottawa, Canada, researchers presented information, according to the BBC, showing that the super strong jaw bones and longer canines developed at a time when dog ancestors hunted in groups.
Have there been changes over time? Yes and no. I would not think that a small Maltese mouth could eat a deer rib, but several students of mine have said that their dogs can do this. I also see more (though still not many) broken canines when gnawing on very big bones.