The big one is the female. The little one (male) will look like its seizing every once in a while. This is the fish submitting to its female. No worries. Dont worry until you see agressive bitting or fins being nipped.
Clowns are weird fish, they may do weird behaviors. Just watch them and if it seems like they are gasping for air or diseased then its a problem. For example, my clowns sleep ontop of eachother in the sand and take a few minutes to wake up when the lights go on. My dads clown fish swim vertically all the time and sleep vertically. My female clown loves to swim sideways everywhere.