Fair comment, but you often hear on here of dogs that will lay in to bigger stuff yet still retrieve rabbits alive. And to be honest I do not disbelieve the people who say it.
TC
If a dog has had a good nip off teeth at a young age then maybe he will just be hard on anything he grabs mate. There's so many different experiences the dog will go through as its growing up. And maybe the dogs confidence will also have a major effect if he's running in half hearted then he's is gonna mangle what ever it is.