A girl who is too young for you has an attraction for you, but you can't do anything about it due to her age. So you "feed the pig" until she is older and then you allow her attraction for you to turn into something.
Origin: Came from the term "Feeding the pig" which is a situation in where you have a small pig that is useless to you, so you feed it and nurture it until it grows to a fat pig in which you then have a use for it.
Tom: Why don't you just feed the pig until she's older?
Bill: Ya man, feeding the pig on this girl will pay off in the end.