Just about every sealed battery (phone, laptop, UPS) will balloon up like this when the battery gets past its prime. Amazon usually carries off-brand replacement batteries which will do the job nicely & cheaply.

FWIW, I've encountered countless Macbooks with swollen batteries, including a few which cracked the touchpad on them. On my own, the battery had swollen so much that the touchpad wouldn't click. New $30 battery fixed it.