If you are small on top, you may want to try the newest swimsuits from Brazil. The Brazilian woman is generally smaller on top that the American woman, and their swimsuits are cut accordingly.
Look for tops with a touch of padding, or an under wire, demi-bra style that makes the most of a beautiful, dainty, bust line. Try to find a top with adjustable straps at the shoulders and around back so that you can tighten and adjust, to help fill out the top line.
Avoid tops with too much fabric or have a poor fit. Opt for separates, were you can choose your top and bottom separately, try ruffled tops or bandeau tops.