The unusual hood in the male hooded seals is on average two times bigger than a football! It has a volume of about 6.3 liters and when inflated it doubles the size of the seals head.
When you see a red balloon instead of a dark grey or black one, this is actually something separate. The red balloon is the nasal septum which they blow out through one of the nostrils. This nasal septum is what normally fills the hood.