Gt, check this link, it might make you feel better:
Depending on who you are, the total fat volume in breast tissue, as opposed to glandular volume, could be as little as 7% to as much as 56%. Basically, there's likely more functional glandular tissue in your breast than just fat. This functional tissue makes up most of the size and a even more of the weight, and isn't targeted by fat burners, so a fat burner would have much less effect on your breasts than it would on your belly fat, or other areas, where the concentration of plain fatty tissue would be much higher.
But judging by how much you run, the fat content in your breasts is probably very low. Belly fat tends to be much more stubborn, and last to go when you're getting in shape. So bottom line is... If it does alter your breast size at all, it'll probably be barely noticeable.