From Pathfinder Howl of the Wild, which isn't enabled.

Toppling Transformation

FEAT 6

Activity reaction

Trigger Your size increases due to a polymorph effect.

You use your body's expansion as leverage to displace a nearby creature. You attempt to Shove or Trip an adjacent creature. For the purposes of determining what size creature you can affect, use your final size after the triggering effect.

Pathfinder Howl of the WildORC