Sometimes it is to do with genetics.
Will probably get berated over this but my DD1 is nearly 16, has a fantastic figure and is 'normal' size (Top Shop size 8/10).
My DS1 is nearly 12 and is very 'small' for his age. Has a six pack (not from working out, but does lots of exercise) and is very slight.
My DD2 is nearly 7 and has an overgrowth condition (Sotos Syndrome) - very tall for her age, 'solid' built but not fat, delayed development.
Looking at my husband (the children's dad, we are no longer together) and me, I would say that my elder 2 children look like they are our children, average (may be a bit smaller) size, average height (may be a bit smaller) but my youngest is tall for her age.
People look at her and think she may be over-fed but she is the healthiest-eating of my 3 and is just 'big'.
On holiday this summer I had a lovely photo taken of my DS1 and DD2 (aged 11 and 6) and their height difference is about 2 inches and they are exactly the same shoe size (my son is a 3F in Clarkes and my DD is a size 3G in Clarkes) and she is 4 years, 4 months younger.
Saying that, if all my children were over-weight (and me and their dad were as well), then understandable but please do not judge my DD just because of her genetic condition.