My grandma has large but my mom...?

My grandma has large s but my mom has same size as I do (36A). I'm pretty sure my g-ma has either a C or close to a C. Anyway, how is it that my mom got stuck with small if her mother has big s? My mom is 5'5 but like stick thin and has been since childhood. My g-ma isn't fat but she's about average with a little extra fab. When my mom was born, my grandma was skinnier than now...