runt of the litter
The smallest and weakest person in a group, especially a group of siblings. (A "litter" is a group of newborn mammals, the smallest of which is least likely to thrive or survive.) Even though I'm the middle child, I've always been the runt of the litter in our family—even my younger sister picks on me because she's taller than I am.
runt of the litter
1. Lit. the smallest animal born in a litter; the animal in a litter least likely to survive. No one wanted to buy the runt of the litter, so we kept it.
2. and the runt of the family Fig. the smallest child in the family. I was the runt of the litter and the butt of all the jokes.
runt
n. a small person; someone whose growth has been stunted. (Also a rude term of address.) He can’t play basketball. He’s just a runt.