One downside of being lean if you have natural breasts (i.e. no implants), is that the leaner one gets, the smaller they get, since breasts are mostly fat tissue (amongst other). As you develop the muscle in your pectorals, the shape of them can change as well.
The leaner you get, there's probably not that many products that you can use to help keep your breast size, or give them the round and fuller shape that they have when you are at a higher body composition. Having higher carbohydrate intake can sometimes help - used to with me for a while at least, but there comes a time when you get below a certain body composition when they are going to stay smaller, no matter what you do (unless enhanced). If you intend on staying so lean that you have perpetually smaller breasts than you used to, but do not want them thus, the option - that most competitors and fitness models choose - is to get implants. If "going under the knife freaks you out a bit", then you can always do things like wear a bra that helps "enhance" your breasts, etc. In the end, what you do depends on how confident you are with your own body, and whether or not the body you are creating is one that you are happy with.
~Rosie~