Hi S. -
My daughter is just shy of 16 months and only weighs 17 lbs. We recently moved so now we have a new Pedi. Both have observed her and she is very mobile, and shows all the appropriate developmental milestones. She has been walking since 10.5 months and I was warned that at that point she will gain VERY slowly - simply b/c she burns it all off.
She was 6 lbs 14 oz when born, but since then has ALWAYS been petite with every well check up. She's shown no signs of stomach issues and until toddler ficklness, would eat EVERYTHING.
Both my old and new pedi have advised me that as long as she keeps trending up over time and is displaying all the right developmental milestones, no big deal at all.
What's funny is that my daughter is so much smaller than all the kids in her day care class, yet she has no issue holding her own, taking food when she wants it and same for toys!
I did breastfeed until about 3 weeks ago. It does seem to me that BF babies don't seem to be as "heavy" as formula fed. I wonder if this is b/c at the breast, a baby can regulate how much they take. With a bottle, they can not, so overeating (and hence more weight) can occur.
Interestingly enough, my daughter has also rarely been sick. At 1 year she got something cold-like that could not be pinpointed and she eventually got over it herself. And at 14 months, she got her 1st ear infection combined w/ bronchitis and was put on antibotics. I only mention this b/c if her being small was an indication of health, then there is no connection - rather it might be the opposite. As a contrast, my 18 month old niece weighs 24 lbs and is almost CONSTANTLY sick from November through the spring.
I suggest you look at a complete picture of your daughter and not over worry about one data point if the whole picture indicates she is doing fine.