I have 2 Dcs, a DD who is 2.5 and 10wo DS
DD always had poor weight gain (EBF to 6m)and was seen at the hospital after she fell across 2 centiles. The consultant said there wasn't anything wrong with her, she was just born bigger than she is meant to be. She was 8lb 6oz, which was 75th centile in the book and is now on the 9th for weight and 0.4th for height. At her 2 year check we were told we had to go back in a few months for this reason (haven't been yet). Goodness knows what they will do about it. It is really difficult when they are EBF and don't gain enough weight and you don't know what to do about it. It can be very soul destroying.
My DS, on the otherhand. Was 9lb 12oz when he was born, he is a lard arse and is still on the 98th centile. No signs of dropping down centiles here, his sister had by now. So pleased.