mine wasn't born the runt but got sick and when i was able to take her home she was much smaller than the others. she turned out a little smaller than the others, that i know of, but not too much smaller. she didn't take longer to grow, as far as i know, as well.
I kept the runt out of my last litter. He was born 7 hours after the pup before him. when they went to their new homes the biggest one weighed almost 8lbs. My runt weighed 4.1lbs. they are 6 months old now and all weigh about the same. premature human babies grow up to be 6'5" males while 8lb. babies are small as adults. Same principle.
I actually soemtimes prefer the "runts". To me, many times they are the spunky ones. They also usually catch up in size to the other pups eventutally, or sometimes exceed them. I would not worry aabout taking a runt, as long as they are confident, and sure.
trudi was the runt smallest by like half and now she is a feisty little dog still about 2/3rds the size of the other dogs in that litter. but she is tougher than the rest. and faster and can jump higher last longer. just all around a better dog!!!
This dog was the runt of her litter but turned out to be the best. I do not think being a runt really determine anything about the dog. As long as it is healthy it is going to turn out to be what ever life wants it to be runt or not.
Siren is not only the runt but the pick of the litter, she has done well in conformation
I can almost bet you Fly was the runt of her litter, even though im not completely 100% on that. She was no bigger than a 2 liter pop bottle when i rescued her around 12 weeks; and now she is 52 pounds solid muscle and smart as a whip!