I know my son is small. He was born at 30 weeks weighing 2lb 7oz. I know he's smaller than a newborn. You know why? Because he's a preemie and preemies are small. When you look at a photo of him, don't let the first thing out of your mouth be "oh, he's so tiny!" I know he's tiny.
When you see a newborn baby what do you say? You comment on the amount of hair, their cheeks, their eyes or facial expressions. You comment about how cute they are whether you actually mean that or not becaue let's be honest, sometimes it's just lip service. We all know "those" newborns.
How about next time I see your kid I comment on something that's super obvious. You show me your kid and I say "Oh, their head is so big!" or "MY oh MY that's a large child.