I am not sure exactly what you should do for the little sow, but I would keep a close eye on her, and feed her well- if she is acting healthy otherwise, she might just be a runt, and always be small.
- I GAVE, dammit!
Usually, the smaller the litter, the bigger the babies. My last litter here was 3, and the one boar was born at 6 ozs. He is now 7 weeks old and 23 ozs. He is a moose.
I have had litters that were quite a bit larger, and a couple of the runts never did get big. Just make sure they are steadily incrreasing, even if only by a little bit each week.