While "more than a mouthful is a waste", it is difficult to find the right cut in shirts for women with small breasts. Too many shirts make flat-chested-women look boyish, or, they hang in a way that looks sad.
Shirts Bad Low-Scoop Neck Shirts
Deep-V-Neck with 3/4ths Length Sleeves
Good: High Neck Tank
Cap Sleeved Boatnecks
Why:
Scoop necks put the breasts on display. If there's nothing to fill the window, it just doesn't look right.
Deep-Vs do the same thing, and 3/4ths length sleeves cut the arm off at the thinnest part, making the over-all effect very skeleton-like.
The other looks give emphasis to key features like the neck and shoulders.