I will see if I have the weights in my book at home or on my spreadsheets when I get home for our litter of 11 that we had in March. When compared to our litter of 7 before there was a notable size difference in them but once they were able to get out on their own, in their new houses where they were not vying for food the gained to where they were supposed to be.
As for keeping the whelping box clean, consider a litter box that you can put in at 3 weeks to start training them where to use the bathroom. Then you scoop like a cat litter box and keeps the room cleaner and makes the whelping room a whole lot nicer place to be.