Adj. Larger than expected, necessary, or appropriate.
John Q. Lardass: I'll have a triple bacon cheeseburger, supersize that please...two Biggie Fries, supersize of course, a two-liter Coke...a supersize chocolate shake....
Southwest Airlines clerk: What can I do for you?
John Q. Lardass: I need a ticket to Dallas.
Clerk: That will be one supersize ticket to Dallas, $850 please.
John Q. Lardass: What!!?
Clerk: Sir, customers must pay for all the seats they occupy. I see you are still gorging on your supersize fries, so don't try that "glandular disorder" crap with us today. Take some goddamn personal responsibility.
Fatass: I'll have a Big Mac meal, supersized, with a chocolate milkshake. Three apple pies...actually, make that a milkshake a Diet Coke. I'm really trying to watch what I eat.