
I would agre in the case of an over large litter, that taking the big strong pups away so that the smaller wones get more milk is the way to go, with possibly topping up the big ones.
In my experience though if you have a week or slow pup, such as can happen after a C section, or perhaps long time being whelped, that the wek ones do well if topped up for a few days to gain enough strength to feed efficeintly. Milk production is based on supply and demand, and as in humans the first threee days of near constant suckling on demand brings the milk in. If for some reason the pups are not feeding strongly, th3en the milk supply is compromised, and th4e pups do not gain at the expected rate and are playing catch up.
I have found that supplement ing at up to half the pups requirements for a few days just get the pup going, and it will then feed more vigourously from Mum in between thereby helping to get a good bitch milk supply going, usually after 4 days to a week these can be phased out.
Sadly I recently had a pup whose sucking reflex was very poor, and even with this approach made no headway, and I ended up supplying most of it's needs, but stil it couldn't get ahead, and I had to call it a day.
A bitch that is well fed will supply the amount of milk required, making less milk for a small litter than a large. My current Mum is doing her 4 babies well, but three years ago her litter of 9 developed at the same rate.
Sometimes pups in a small litter are heavier than a larger litter, as they were larger born, and therefore more vigourous to start with, but by 8 to 10 weeks any such differences are evened out, and it can be disconcerting to find that the fat pup finishes an average adult, and the fairly small one just keeps growing and makes top size.