Yes, be careful.
Little ones can gain pretty easily. I prefer a lean dog over a tubby one.
Jackson is a healthy active 16lbs and eats 1/2 cup per day of Acana kibble (1/4 in the AM, 1/4 in the PM, both times about a tablespoon of wet food added). Acana is about 430 kcal/cup so it's a calorie dense food meaning less of it needs to be fed.
For a good like Natural Balance, I believe it's less calories, so if I was solely feeding NB, I'd probably up the amount to 3/4 cup per day for Jackson. I read somewhere that NB Sweet Potato and Venison is around 368 calories per cup -- it's not on their website though, not sure of the other formulas.
I just go by his body condition and how much activity he's had. For example, last week on Sunday we did about 3 miles of walking, 2hrs at the dog park, and 3-4hrs of running around my dads fenced in yard and just being outside. He was REALLY hungry this day and I could tell -- so I fed him a bit extra.