I have a litter of two pups which are 8 days old, average birthweights for my breed are 6-8 oz's. One pup was born 8 oz and is now 14 and the other was born 4 oz's, both girls. The tiny one hasnt put on any weight it keeps going up to 5 one day and 4 the next. I noticed it has a slight swelling of the eye so have made appt at the vets for this evening. Mum and the larger pup are doing fantastic, the smaller one is doing everything the larger one is apart from its weight, im doing everything i can for this girl i suppose nature will decide but its my first litter so im confused.
Not looking good is it :(

Your pup may have an infection which antibiotics may cure. The slight swelling in one eye could be an indication of an infection developing inside the closed eye. This is not unusual in a pup that could have picked up an infection during birth. If this is what has happened there should be no long term damage to the eye and should  easily be cured.

Are you sure the smaller one is actually able to suck strongly enough to get a proper supply of milk and it is getting enough fluid intake?

Yes it is strong sucking, she looks very well chirping, twitching, pooing etc

With a good suck reflex, a stable warm environment, a mother with plenty of milk, antibiotic cover, vigilance for dehydration...................... then little 'uns can and do thrive perfectly well.

My theory ( and it is just my theory) is that some low birth weight pups are slow to develop the complete enzyme compliment they need to fully digest the milk they take in.
I feel this is proven by the way they suddenly take off when all other factors are the same and, although their littermates are streaks ahead, they catch up.

Good luck

Thankyou both for the replies, it will be interesting to see how it goes.
Will let you know of any improvements.