It is a larger variation of a normal Krabby Patty. If the customer requests it, it can also be batter-fried twice.
Role in episode
Harold Bill Reginald Scott orders a King-Size Ultra Krabby Supreme that was also batter-fried twice with mayonnaise at the beginning of the episode. He then proceeds to eat it and squirts the mayonnaise bottle into his mouth, which prompts Squidward to express his disdain for Krabby Patties, beginning the episode's conflict.
This was recreated in an episode of Binging with Babish.