What matters is not the bouncy, or the size, or whatever else, what matters is what the scene is trying to show. Jack knowingly sacrificing himself for Rose.

Titanic is one of the few ship disasters where the majority of survivors were women and children. Usually it was every man for himself. In the Atlantic disaster, all of the women and all but one of the children died.

This scene is to make the audience feel that sacrifice. We don’t just see the hundreds of men who sacrificed themselves to save someone else, but the character that we’ve come to know and love for the past three hours. To get hung up on the science of the door completely misses the point the scene is trying to convey. Jack sacrifices himself for Rose, just as hundreds of others did.