It was a normal-sized patty, but Mr. Krabs stole a growth serum from Sandy and administered it to the patty, making it grow.
It looks just like a normal Krabby Patty, only it is much, much bigger.
During its growth, it ate the Krusty Krab, and Cashy. It grew so large, it covered up, and ate most of the citizens of, Bikini Bottom. It was finally destroyed after SpongeBob Squarepants absorbed it into his body, making it 'spit out' everything and everyone it had 'eaten'.