Breasts are associated with females and that in itself if titillating. Large breasts, therefore, are sexy. By definition, they are characteristics sported by sexually mature females, so they are sexy. That also means, by definition, if they are smaller, they are more characterized by being owned by juveniles. This means smaller breasts give you a younger/cuter look, whereas larger ones give you an older but sexier look.
That being said, many guys love a cuter looking girl. Look at a lot of Asian girls… they are small… but they also have many fans.
That being said, do you like a smaller look? Some like the idea of a cute girl. A petite framed girl who likes to cuddle and you can carry in your arms.
Besides… if you change your mind… they get bigger with pregnancy. If they were very small, to begin with, they tend to get bigger permanently.