Nico Robin got her hands (or in that case, her boobs) on a devil fruit, specifically a Grow-Grow Fruit. Her calm deminer would blend in with her dominance turning her into a power force. She would grow bigger and bigger as she towered and belittled any person (literally). Her thighs got thiccer and her boobs got much larger, close to ripping her bikini top to shreds.

The devil fruit still had many bites left which means there were still higher heights Robin will expand and grow to by time.~