Every birth is different and there are many factors that determine how healthy the offspring will be. If a mother cat is taken care of, then it’s quite possible that all her litter will be the same size and healthy.
The runt is usually weaker and might not be able to develop properly. It might also have congenital defects. Of course, some runts manage to survive, especially with the attention they get from people.
Life as a runt might be more difficult, especially at the start. Because of their size, runts are often very noticeable and while this feature might not be so appealing to their mother, there are many people who’d gladly adopt one!