Changing a unit's mass ingame

[AgentPaper]

Does anyone know of a way to do this? I've got my unit spawning triggers (TD) set up to automatically set the unit's HP to the correct amount, but I can't seem to change their mass in any way...I need their mass to increase as levels go higher, but which unit is at which level is randomized so I can't just set the masses in advance. Mass doesn't seem to be a unit property changeable by triggers or behaviors as far as I can find...any ideas?

Edit: Oh, and the reason I need the mass to change is so that towers that push/pull units have to be upgraded to be effective against higher level enemies.