If something is stuffed, it's packed full of material, like the giant stuffed rabbit you won at the country fair. If a person is stuffed, she's had too much to eat.
You might sit back after eating an entire pizza and murmur, "I'm stuffed." If you're stuffed up, you've got a bad cold and your head is stuffed with mucous. When objects are stuffed, it's usually because they're full of stuffing, like a stuffed armchair or a stuffed animal. It was once common to insult someone who was pompous but ineffectual by calling him a "stuffed shirt."