That big ball of spikes is George, an extremely obese hedgehog (he tipped the scales at almost 6 lb or 2.7 kg) found by a wildlife aid group on the side of the road along with his friend Mildred, which is normal-sized.
Link (with video)
See also Neatorama's Top 15 Amazingly Fat Cats
Comments (8)
They implied it was really heavy as well, so unless the air trapped under its skin is particularly dense, that would seem not to be the issue.
That's the whole idea behind the Simpsons, though. The point of the show is not the story, but the characters. The story has become secondary to the situations the characters get in.
I like your thought about the show - I just wish the singers had achieved what you hoped without being so unbelievably annoying.