Why did women show their decolletage so much in the past?

World According to Garp, IIRC.

Just read War and Peace, and in a section describing going to the theater comments were made about the amount of bosom women showed (Pierre’s wife Helene, I believe.) That novel was set in the 1st 2 decades of the 19th century.

I’m having trouble understanding the text. I request visual aids.