Yes, exactly. It's suggesting that someone is not truly fat (from over-eating) but it's an illusion, their large bones just make them look fat. "I'm not fat, I'm just big-boned."

Literally "big boned" means having a massive bone structure. In the movie it would refer to a fat person. A lot of overweight women make an excuse about their weight claiming that they have "big bones."

"Big-boned" is a humourous and falsely-polite way of saying that somebody is big, fat, large, enormous.