
Sounds absolutely fine to me. Size of a bitches milkbar has little to do with amount of milk. I had a very flat chested bitch rear 7 pups with them gaining really well.
As long as the rate of weight gain is slowly increasing and pups happy then all is well. If the rate they are gaining at starts to level off/drop I start introducing weaning foods any time after their eyes open, but normally start at 18 days so that by 21 days they are having 4 taster meals a day (I like them on solids when I worm).
I have never ever had pups double their birth weight by a week sometimes by 10 days.
Mine are mostly born at 12 - 16oz (though I have read in book of the bitch 10 - 12oz as average) and if on larger side they loose weight the first day, regain second or third and then gain after that, at about an ounce a day.
So I am happy with 4 - 7 ounces first week, then expect at least an ounce a day second week, and it increases after that to about a pound a week to five weeks.
I have had some really greedy litters with heavy pups, and have had less hungry slower gainers, and have found this had little to no effect on eventual adult size and that most pup were similar weights by 8 weeks (9 - 11 pounds though have had some heavy weights.