ANDY'S ANSWER: Puberty is a process that takes several years. I am assuming you have started puberty, so you've likely seen some changes already. Some girls will see a lot of breast development all at once and others will notice it that it happens more slowly.  The other thing is that not all girls develop the same size of breasts. Genetically, you may have larger or smaller breasts than someone else when fully developed. Don't worry. Until you have completed puberty, nothing is finished.