I know that this will most likely never happen because of the size difference, However Lawler used to fight at MW and Rumble used to cut down to WW so its not theoretically impossible. Both are known for their extremely hard punches / scary appearance alltogether. I would give it to Rumble, one of the most dangerous strikers in the UFC right now hands down. There would be no danger of takedowns / submissions from Robbie so it would be a stand-up dog fight. Robbie would get TKO/KO'd even though it wouldn't be the optimal weight for Rumble to fight at his best effectiviness.