Among several small differences the most telling is the lack of ear flaps in the seal whereas the Sea Lion has ear flaps... Sea Lions are, on average, quite a bit larger than seals...
Beside the external ears, Sealions can turn their hind flappers (feet) forwards, so can get about quite freely. Seals can't turn the hindfeet forward so for them to get about hey have to bobble forwards.
I'm afraid the size of the animal is irrelevant. Elephant seals are enormous, the males easily reaching 16'