It was a regular patty, but until Mr. Krabs stole a growth serum from Sandy and administered it to the patty, it started to grow.
It looks just like a normal Krabby Patty, only it is much bigger than other Krabby Patties.It is also covered by grease , probably because of it's large size that cannot be seen into a regular one.
Role in series
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 absorbed it into his body, making it 'spit out' everything and everyone it had 'eaten.'
The Giant Krabby Patty itself could be a reference to the old B-Movie, "The Blob".