
I have never found any of my pups, except in a very small litter double their weight in 8 days, about an ounce a day the first week (between 5 and 10 ounces in the first week) is fine. I suppose if they start at 6 ounces then they will double their weight, but my 12 ounce ones have rarely doubled theri weight that quick.
Puppy weight in the nest also doesn't seem to have too much bearing on ultimate size. I have had whopping pups turn out average, and average size pup grow big!
If the pup looks plump and shiny, and is active and content, and feeds efficiently (the tail wagging is a good indicator tha the milk is flowing well, and you can hear them glugging) then I wouldn't worry too much. If you find the weight gain starts to slow that will be a sign that you may want to start them on solids. I have started sloppy solids anytine from 10 days to 3 weeks of age, depending on how Mum is doing, and the size of the litter.
If you feed the bitch in her bed you will know when to try solids, as the pups will start taking an interest in her food.