Its called Gross muscle hypertrophy caused by a mutation in the myostatin gene--- this disorder causes severe and painful muscle cramps in the dog---
I hope breeders are NOT cashing in on this disease at the expense of folks who have no clue what they are getting into---
---IF SO---IT MAKE ME SICK.
Bully Whippets are 100% healthy and normal tempered, the gene is a "mutation" yes but that's what every gene is.
It only a very slight chance at hip displaysia, something most dogs over 35lbs risk getting. It does not lead to muscle cramps (guess you assumed big muscles = instant muscle cramps?) or "severe pain."