exercise doesn't really increase breast size, but if you build up the pectorial muscles which lay behind the breasts you can lift your posture and push the breast tissue forward, giving you a more pleasing shape.
You might want to try:
Barbell flat-bench press (if you're not already doing them).
This is the overall best exercise for producing chest fullness.
Incline dumbbell flyes
Emphasizes inner chest (i.e., cleavage) and provides lift.
Twisting dumbbell bench press
Lie on a bench as if you were doing regular dumbbell bench press. Movement is same as a regular dumbbell bench press, EXCEPT that as you bring the dumbells up into the air, rotate your hands so that your pinkies are together at the top (hands facing backwards). On the return movement, rotate your hands again, so that when your hands are at the bottom (near your chest), your thumbs are pointed towards one another again, as at the start.
This exercise emphasizes inner chest muscles and pulls pecs (and hence, breasts) together, again emphasizing cleavage.