'It's Always Sunny' Actress My Ass Is UNREAL

5/16/2013 10:07 AM PDT BY TMZ STAFF

Mariann Gavelo has a huge ass. We're talking Kim Kardashian style HUUUUUUUGEEE ... and it was all out on display at a party in Hollywood last night.

Gavelo -- who appeared on "It's Always Sunny in Philadelphia" (the "Thunder Gun" episode) and in Will Ferrell's "Casa de Mi Padre" -- hit up the Maxim Hot 100 party with her badonk in tow.

Besides her dumper, Gavelo's a pretty petite chick, so we gotta ass -- is there ANY CHANCE that butt is real???