Hi everyone, hope you can give me some advice. My small breed dog (not toy) had a small litter of 2 who are two weeks old tomorrow. This is a small litter for the breed (they usually have between 3 and 7). She had a difficult birth which involved a trip to the vets as one of the puppies was so big she got stuck. They are doing very well, doubled birth weight within 6 days, eyes open, moving around lots.

My concerns are (and please forgive me for sounding niave). At birth, the biggest puppy (who got stuck) was slightly over the average biggest weight banding for the breed. The smaller puppy was bang on middle of the road. We are concerned that the larger puppy is getting very very very big. She feeds constantly and is significantly bigger than her sister. I move them round the teats and always put the smaller puppy on the hind teat but the bigger one is still so big! Is it possible for a puppy to be overweight? Should I be doing something differently? She moves around often (neither can stand at the moment). I read an article about chunky puppies becoming swimmers and I’m terrified

It is unlikely that she will become a swimmer. If you think about singleton pups, they have sole access to the milk bar and do not become swimmers.

She's probably just a happy foodie. Once they are up and about she may become more interested in her surroundings and exploring over eating. When you wean, the smaller pup may start to catch up.

I once had a two pup litter and the bitch pup was a proper foodie compared to the male. At 8 weeks, she weighed a whopping 10kg and her brother, a much better 8 kg. We thought she'd grow to be huge but fast toward 5 years and she is on the petite side of the breed standard whereas her brother is huge!

Just keep an eye on them and doing what you're doing.

Thank you so much for your reply. You’ve made me feel so much better. This is my first (and last) litter and I think I’ve completely terrified myself reading all of the things that could go wrong. I have a mentor and parents who have bred in the past and they seem so relaxed about everything and I’m a nervous wreck!

I can vouch for big singleton puppies not becoming overly huge as adults.   We've had 2 singleton litters, and the only problem I had other than feeling so sad for them once mum wasn't in with them 24/7 (at which point I'd stuff used socks for the puppy to be with) was to make sure all the teats were being used at some point.   Neither became or were swimmers. 

If you don't see swimmers fairly early on, you won't have this.

Just to add, put the smaller puppy on one of the middle teats as you may find it can't cope with a big rear teat.

http://leemakennels.com/blog/dog-breeding/how-to-save-a-swimmer-puppy/
Not that you have to take onboard everything that you'll find on the Internet, some of what's in this article may help you understand what a swimmer is, and how to treat.