
I have 5 puppies of 4 weeks that weigh around 4 - 4 1/2 lbs.
They are getting two handfuls (about 4oz) of soaked complete puppy food, this amount 4 times a day, between them.
I would say once soaked it is probably equal to two thirds of a tin of food (but I wouldn't use the tinned other than as a part of their diet). In addition to this they get a 1/4 pint of puppy milk formula, so a pint between the 5 of them over the day.
They feed from Mum after each meal and whenever she cares to feed them in addition.
I imagine you are not feeding yours enough.
I always make up enough food for there to be a little left for Mum to clear up.
If the next day they clear each meal then I increase it slightly the next day.
Vets are experts in the diagnosis and treatment of animal disease, and unless they breed themselves are not the best person to advise on puppy rearing.
This advise should be coming from your breed mentors (usually someone senior in your breed who is guiding you, often your bitches breeder and the owner of the stud you have used).
Mine are probably eating about half the amount of an adult of my breed, and by 8 weeks will eat as much as they do, and by 12 weeks about 25% more than an adult.