Misconceptions regarding the 'runt' of a litter
"In a group of animals, a runt is a member which is smaller or weaker than the others. Due to its small size, a runt in a litter faces obvious disadvantages, including difficulties with competing with its siblings for survival and possible rejection from its mother. Also in a domestic dog litter, most puppies would have to make room for the runt to get milk from the mother. Therefore, in the wild, a runt is less likely to survive infancy."
The above information from Wikipedia is quite right, but. Having bred litters I can quite easily say, this so called runt can change throughout the litter depending on the puppies age. It's easy after the initial birth to label one puppy the runt. But on the other hand, wait until they are a week old and your opinions of which puppy in the litter is the runt will more than likely be quite different. As puppies develop over the first few weeks of their lives they change so rapidly it can be hard to tell a runt from the rest.
If a lot of time and effort is put into breeding a litter you are less likely to have a runt as nature does not take it's course and this runt does not tend to lag behind the rest. You weigh your puppies everyday to make sure they are gaining weight evenly. Most puppies feed every two hours from birth so their weight can fluctuate throughout the day, you may find one behind in the morning and ahead in the evening. If one slips behind for a day you work hard to make sure it has plenty of milk the next day to catch up to the rest of the litter. And now you have worked on this puppy he/she is now stronger than another one, so you do the whole routine again with a different puppy.
Monitoring in this way can decrease the chances of a so called runt. In the wild this would not happen and meaning rightly so, that the runt faces obvious disadvantages completing with other siblings for survival. Due to the lack of nutrients from minimal feeding this puppy becomes weak and all in all there probably would not be a great outcome for this pup. With supervised feeding/weaning this is very unlikely to happen.
At the end of the day, you will always have one puppy be the smallest in a litter, it's a fact! Just the same as you will have one puppy the largest. No one has ever bred a litter where all the pups weigh exactly the same throughout their lives. As they develop in the first few months of life, the females will almost always be overtaken on weight by the boys. This should not always be looked at, that they are the runt.