Breast question

So I have a breast that is slightly bigger then the other. I know that it's normal but I was wondering why do you need to favor the bigger breast when buying a bra? I hate it but do it because I read that you favor the bigger breast but when I wear my bra my left breast that is a cup smaller has this small space and I really don't want to put in a pad but like I mention before I want to know why? And any suggestions of what type of bras are good to wear.