Is already 15.5inches and still looks like he is growing. He comes from a very good breeding line. If anything he was atleast halft the size of his sisters. Any ideas as to why

Although size of parents and siblings can be a guide to size that an individual pup may be .... some can surprise LOL.
My Lab was the smallest in the litter but grew taller than her Mother and siblings.

Hi Emma, I have PM'd you.

"he was atleast halft the size"
If he was half the size of his sisters, and he was already 15.5, I think maybe the breeding needs to be reconsidered, because thats a huge litter.  Unless you mean at least twice the size of his sisters?  To have a whole litter at 15.5 inches at only 16 weeks is pretty unusual if he's from a good breeding.  I heard once that if there were say, 4 puppies in a littler, 1 will be undersize, 2 will be in-size, and 1 will be oversize.  They usually vary a lot depending on the individual dog though.  But at 16 weeks, I would still count on him growing more.  Males often grow until 6-8 months depending on the lines, so he should be a very big boy.  I have an 18.5 inch boy, and he's the best dog I could ask for, so have fun with your puppy!

I think the poster meant that when they got him, he WAS at least half the size of the others. but has now shot up. :)

Correct me if im wrong!

No matter how good the breeding you can always end up with something that may not look quite how the breed should be.  I remember about 3 years ago having a litter of Pomeranian's, the owners came back a few months later with one of the pups that we bred and he was about 4 times the size a Pom. should be (more like the Sharon Osborne Pom) and we'd never had one so big :d  His colour was a deep orange sable too, with a lot more black than you wd. normally see, he looked like a baby GSD !  This pup was from one of the best lines at the time too, so like everything in life you can't always guarantee on something turning out the way you expected it to;.

lol a friend of mine bred a litter of shetlies from correct sized lines & parents, one beautiful male won a class at a limit show & was placed as a rough collie he is so big the test of the litter are quite normal sized