How do principles of natural selection help explain why there are more deaths among babies whose birth weight is low?

"Natural selection is the gradual, non-random process by which biological traits become either more or less common in a population as a function of differential reproduction of their bearers." - Wikipedia

Babies with low birth weight are more susceptible to dying because their bodies are too small to weak to sustain life. Reasons for low birth weight are varied and depend heavily on the health of the mother. It's not really a natural selection process.