Unless your kid is losing weight or malnourished, I wouldn't sweat it too much. All three of mine are fairly light. My eldest is the "hefty" one. She has had a voracious appetite since day one, and would eat until she puked when she was an infant (I learned when to stop feeding to avoid the vomit). She always ate a good variety of foods. She was 19 lbs at a year old. She is now 17, 5'4" and 105.
Middle kid was a picky eater until about age 8. She now eats a normal amount and a good variety of foods. We kept getting those "your kid is underweight" letters from school until a couple of years ago. At a year old, she was 18 lbs. She is now 15, is 5'4" and is almost 100 lbs.
Youngest was heaviest of the three at birth - 7lbs, 10 oz - but lightest at a year - 16 lbs. Some days she eats a fair amount, other days she doesn't eat that much. While the quantity has improved with time, the quality/variety of foods has not. Doctors told us we should not feed her veggies or fruits when she was an infant since she had poor weight gain. She doesn't eat many veggies or fruits now (at age 8). She eats a lot of meat, pasta and dairy. You would think the kid would be chunky, but she is 1st percentile BMI (weighs 41 lbs).
So, as Howler said, don't worry too much. My youngest was deemed "failure to thrive", but the doctors could not find anything wrong with her. She was on a very high dose of steroids for several months when she was an infant, and that can have temporary effects on growth (as long as until around age 10), but I think her growth/weight gain is very similar to middle kid. We're not stressed about it. We have plenty of stuff to stress us - right now waiting for college decisions for the eldest (and financial aid!).
