They had breasts, but they were often comparitively small. Women have been getting steadily taller, stronger, and bustier, since the 1970s.

They faked it, with corsets, as cinching the waist tighter makes the breasts more prominent. In the 1920s, they actually bound their breasts and starved themselves down, as flapper style was inherently androgynous (androgyny was all the rage, see Weimer). A lot of Nazi propaganda pointed this out, and androgyny went out of fashion for a few decades, with women like Marilyn Munro and Sophia Loren rising to prominence, but it came roaring back in the late 1960s, as women become feminists and careerists.

For a long time, the ideal bust was an A or a B Cup and many women actually bought those sizes, even though they didn't fit. It is only recently, that bra manufacturers actually produce other sizes, but even bust circumference measurents indicate busts getting steadily larger.

Turn around after 2000, with more shapely females coming back into style, as other "curvier" ethnicities become more "visible" in the media, like Hispanics, African Americans, and Slavs.

Modern supermodels, like Miranda Kerr, tend to have curvy chests. That is thanks, in part, to Victoria Secret wanting to sell bras. Flat-chested women don't need bras.