I guess every individual is different in terms of metabolism and lifestyle choices, but it did standout how much bigger he was than the rest of his family members. IRS was thick when he was older and doing the IRS gimmick but that was solid muscle and in the 80s he was in shape. Bo Dallas was a rail. On his mom's side Blackjack Mulligan was big and muscular and Barry Windham was tall and chiseled. Bray was just always bigger than them. Billed at 285 and it was not 285 pounds of muscle - his Husky Harris ring gear showed that. The Bray Wyatt beard and outfit hid it better. But his body really stood out in contrast to his supremely athletic family on both sides (that said, he could still perform in the ring and was athletic for a guy his size).
Maybe he was always predisposed to that by metabolism, or the heart defect led him to carry extra weight as much as gaining weight affected his heart function. Or he possibly didn't have the dedication to physical training that everyone else did. Either way, it's a shame he had to go through that.