There are no hard and fast rules, honestly. And how are we defining "fit" and "fat"? Someone who works out vs. someone who does? Yeah, your physical looks aren't always an indication of how much you work out.
No it is extremely common to see fit, thin guys with heavyset girls. Especially as long-term couples (married with children). This is so common as to be far more than "an exception." It's probably most prevalent among couples in their late 20s and up. I've noticed it in multiple countries; culture seems to make little difference.