C.S.
I wouldn't worry about it. My kids were small for their ages. As long as they're hanging out around the same area on the growth charts I wouldn't worry. Every child is different. My babies were small, my SIL's babies were all huge (3 month old is 15-18lbs.) everyone of them is within the ranges of normal. Just think how much easier it is to carry around and pick up a 14 pounder rather than a 25 pounder. :)
C.