Is it a birth defect to have small chin?

Depends. A significant small chin that impedes breathing and normal suck/swallow mechanism is called micrognathia and is part of an array of birth disorders.
Small chin. A small chin isn't a birth defect, unless its part of a syndrome. A small chin is a variant of normal.
Maybe. If a small chin is related to an abnormally small jaw bone structure, it can be a birth defect. Sometimes, if the jaw is very small, it can affect a baby's ability to breathe normally.