Your train of thought is just wrong. For one thing, weight has absolutely nothing to do with lifting Mjolnir. It doesn't matter how much the hammer weighs or how strong you are. It can only be lifted by those deemed worthy, so I have no clue as to why gravity would have any kind of relevance whatsoever. Only real answer is simply very poor writing, and 90% of the time that's usually what you get from Jeph Loeb.
With that being said, Hulk can become so strong that one punch from him would destroy the universe, hes still not gonna lift the hammer.
End of story.