Although it was the first time when BV having his Bottom so Big and Heavy. But it was mostly alot of fat inside him that is mostly suitiable for him and his Species [Because he was technically an Dream Sheep, he's mostly interested on gaining weight].
And he had his Wife, Lammy who also has an Heavy bottom that really contains alot of Fat to help him to get used on any situations to have an Heavy Bottomed Body like this.