there is no such thing as a seated squat, for the same reasons there is no such thing as a seated one leg stand.
i assumed you are referring to the leg press machine, ie something like this:
http://www.technogym.com/gb/viewdoc....get=commercial
if so, the mechanics of the movement are different and there is no weight supported by the upper body so the abs are cut out of the equation: they are not meaningfully comparable to squats. if not, link?
also i am gonna call you out as a liar for your claim you could squat 500lbs. at best you are not lying, just deluded that a 2 inch curtsy with 500lbs across your back counts as a squat. given you dont appear to know what a squat actually is, this is possible