Cody Simpson -- Patrick Schwarzenegger's Fooling Everyone ... He's a Party Animal

Miley Cyrus and Patrick Schwarzenegger took Miami by storm together, but don't get it twisted -- one of them is fueling that party train more than the other... according to their pal Cody Simpson.

The Aussie pop star was shopping at Cotton On in Santa Monica, and talking about his New Year's party plan (he's got a great one) ... which got us on Patrick, Miley, and Cody's raging times at Art Basel last week on South Beach.

We asked who goes harder in the club -- and he backs up what TMZ first told you a couple weeks ago.

You gotta hear Cody gush about Patrick though -- bromance in full bloom.