My daughter is also a small to medium size baby. She only weighted 6.4kg when she was 4 months old. When she was counting the weight, a baby who was 3 months old beside already weighted 6.7x kg already. But I am not so worry.
I agree and the nurses also say that as far as baby is fine and the development is ok, with no sudden drop of the weight, no need to be so worry. Of course, a bigger size baby should be easier to handle and feel like more strong.