We had this kind of issue with our younger daughter - started putting on weight at 8 or 9, having been quite small and skinny previously. She didn't seem to be constantly snacking, just enjoyed biggish dinners.
My mother started to comment on her weight, but having had experience of people with eating disorders, I was reluctant to make it an issue with daughter.
As she got older, she was aware of weight issue herself and still struggles to maintain a healthy weight, even though she has a good diet - just eats a bit more food than she needs.
If you have more than 1 child, it's hard to control what they all eat. Years later it emerged that older brother would get biscuit tin down from high shelf for daughter!