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
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.