Large breasts are one appearance characteristic that is nearly guaranteed to please almost all men, after a pretty face. A big bust is a tremendous advantage when dealing with the opposite sex. Women who already have large breasts tend to play down this advantage and may not even be aware of it (they may not realize the extent to which their breasts are responsible for their attractiveness to men), but to women with very small breasts, it's often all too obvious.
Although many things about a woman can be attractive to a man, breasts are so consistently so that they are often a key target and fixation for women worrying about their appearance. A great many women spend thousands of dollars having their breasts enlarged, but relatively few large-busted women spend money to have their breasts made smaller (unless they are pathologically huge), which is a tacit acknowledgement that big breasts are an advantage.
However, while big breasts are always an advantage, small breasts are not necessarily a disadvantage. Men notice large breasts, and having big breasts works in a woman's favor—but having small breasts does NOT work against a woman. Big breasts are positive, but small breasts are neutral, not negative. Men may pursue women with large breasts, but they do not reject women with small breasts. So breasts always either work in your favor, or they have no effect at all. Additionally, small breasts with a nice shape are just as sexy as large breasts, for many men.
You can always have your breasts reduced, if you want. However, if you're like most big-busted women, you'll complain about it periodically, but you'll never actually have them made smaller. They may be inconvenient at times, but (with rare exceptions) they have more advantages than disadvantages.