My first Lab, my beloved Bess (BF, AKC, bench line, 55 lbs., 1967-81) was the runt of her litter.
My present Lab, my beloved Puff (YF, AKC, field line, 63 lbs., dob: 8-'01) was the runt of her litter -- at 9 weeks age, she weighed 6 lbs. to her littermates' 12 lbs. At 270 days of age, she weighed 52 lbs.
I prefer a smalerl Lab.
Puff [YF, AKC field line (from competing HT/FT breeder) 62 lbs, dob: 8-'01]
Well don't feel bad. I just weighed Sami and she'd lost 2 1/2 lbs. She's 10 months old and 52 lbs. She's a shorty too. Almost without fail, this is what I hear at the dog park: "she's just shorter (or smaller) than all the other labs I see" :
Most runts that I have seen are only smaller in their "early" life.... while they are nursing and are unable to compete with the larger pups for "feeding space". In fact, most runts that I have been around end up being large dogs. My neighbors had a golden retriever that was the runt of the litter and was still the biggest golden I have ever seen. This dog would stop cars in the street (literally walking out in front of them and stopping) and just stare over the hood at them while they would blow the horn. I would just stand there and laugh while the person was looking at me like "get the dog from in front of me"..... hey, it wasn't my dog ;D.
I wouldn't worry about the size of your lab.... every dog is different.