She could still be purebred even with the large feet.

Our Jess has big feet too and she's purebred. Neither of her parents had big feet mind. I'm not sure where she gets them from. The beauty of the variable working strain BC, lol. Her hind feet look especially strange because they are so much wider than her pasterns, they look like clubs on the end of her legs, haha.
She has big chunky claws to go with them. I have to clip them regularly because they get too long otherwise. When Mist was alive, I used to do her claws first although hers didn't really grow that long for the most part but I'd definitely notice the different in their feet and claws when I moved onto our Jess.