Correct, it's from the gene doubling, but the problem with Bully Whippets is you can't treat them like a normal whippet. You have to treat a B.W. like you would treat a larger dog like a St. Bernard.
ie, don't take them to the speed or agility courses, don't play endless frisby with them, don't sprint with them, etc. All stuff that's great for normal whippets, but aren't ideal for the B.W.'s frame.
That is not true in the slightest because a St. Bernards heart and lungs are proportionate to its body mass, a Bully whippits, heart and lungs are proportionate to a normal sized whippit, its not hard to understand how this could be a problem, Im not an expert but after reading this its pretty easy to understand.