Nub cannot actually lift a dead Auroch.
They weigh thousands of pounds. He can't even drag one. Probably an Auroch could drag a dead Auroch, if he tied them together, but it couldn't do it at stampeding speed and that plan obviously ends with a dead Nub.
Nub owns exactly one magic item, a Type I Bag of Holding.
He has no idea what it's standard price is, he didn't buy it at a store.
He's had it for a week now.
Nub barely understood the concept of weight to begin with, and having a magic bag he can put stuff in that never weighs more than he can carry was not helpful to his developing a better understanding.
It's not that the bag has never been full before, but it's always been full of many things. Nub would just empty it out at home and fill it again with new things.
Nub didn't think his bag had a weight limit, or a volume limit. He thought it had a count limit, and only that he did not know what that limit was because he hadn't bothered to count it.
This is the first time Nub has tried to put a single very large object in the bag and for no discernable reason it isn't working.
Also, the body is too big and doesn't fit through the opening.