My DS was (and still is) a slow weight gainer. At his one year appointment, DS weighed just over 18 pounds and the Community Health Nurse sat me down to discuss what I was feeding him. Yes...she used THAT tone. For the record, DS eats like a little piggy but is also extremely active.
As long as they are happy and have enough wet and dirty diapers in a day...I wouldn't be too concerned.
Long, blonde, 3a/mostly b hair.