Without seeing the full shirt, there seems to be at least two types of features that a top could have that would expose the side of the bust. One, it could be sideless or two it could have what appears to be called a "drop armhole" (also called "dropped armhole" or "low armhole") which seems to be used for tank tops and some dresses. Sideless would have no side for the top, except maybe strings to hold the front and back together and the portion that goes around shoulders and neck. A top with a drop armhole features a large armhole that extends anywhere from 1/3rd to maybe 4/5th the length of the top. For a character with a large bust, either type could expose the sides of the bust.
