First of all, my niece is easily 12 kg and is 13 months old - she weighs about as much, or a bit more, than my DS who is 18 months. They are such different kids - she's not walking yet and DS was walking at 10 months. DS still has breastmilk as a large part of his diet, she doesn't, but still is BF'd. Neither of us care where our kids sit on the charts!
Really, you know your child is doing well, so it's a matter of training yourself to deflect the comments of these eejits