ive heard that most of the time when something in a 3d game gets too big it turns unto polygons so im not sure that they can fix it
Its more that the polygons are too apparent like youre zooming into something so you see imperfections clearer. Itll depend on how the models are handled. Not sure if unreal deals with blenders shape keys but it shouldnt be that bad to modify and replace all the models if you just give the belly more polygons.
I would offer help modifying or remaking these models though.