Mass Manipulation – The ability to increase or decrease mass in an object.
Harry Leland (Marvel) is a supervillain that may be portly but he can take on Wolverine (Uncanny X-Men V1 #132).
Kono (DC) has the power to shift mass through time, thereby making objects more or less dense. (Who’s Who in the DC Universe #1).
Iron Man (Marvel) gives the seemingly invincible and already massive Crusher even more mass so that he plummets to the core of the Earth in Tales of Suspense #91.