Adjective
flat-chested (comparative flatter-chested or more flat-chested, superlative flattest-chested or most flat-chested)
(of a girl or woman) Having a flat chest; having small breasts.
flat chested
It's impossible to be more flat-chested than I am. Candice Bergen
Not that it was Twiggy's fault, but the ubiquity of her image created a sense in young women that to be stylish meant to be skinny, flat-chested with an ingenue face and straight hair. Susie Orbach
I used to design for my body, which was flat-chested with big hips. That's why my clothes were tight on top with full skirts. Betsey Johnson