My girl (almost 3 years old) is slim all over, but has a pot belly, too. Sometimes it looks huge. It's hilarious because she's got this skinny little tush and slim hips, long and skinny legs, narrow shoulders, slender arms, a thin and narrow chest, and this bulging mound of a belly.
It's pretty gratifying, frankly. At least the calories are showing up somewhere. (And she does get pudgier before a growth spurt, too. Her cheeks look fuller, her belly is big and seems more solid, and then she wakes up taller. And more lean for awhile. The pot belly doesn't go away completely, though. Or at least, it starts right up again.)
Having a pot belly doesn't really keep her pants from falling down, though. Cloth diapers are our only salvation in that department.