I've actually tried a natural product for that by Isosensuals. I have one breast an entire 2 cups sizes larger than the other, so for years I was really self conscious about it. I tried SO many boob products it's pathetic. I wasted a lot of money, and if you want the truth, you're really not going to find any product that truly increases your breast size drastically or for the long-term. The ONLY time I ever seen results was when I used the Iso Sensuals Enhance lotion, still have some, but I no longer beat myself up about my one big boob and one small one. It was too exhausting and I finally just grew to appreciate them.
However, I did notice a subtle fuller look to my smallest breast. Unfortunately, the stuff is incredibly expensive considering it's necessary to never stop using it (not even for 7 days) or you lose everything you gained. I was really unhappy to learn that, but it's just the way it works.
I used that blasted cream for months and months and I guess I had just reached my "peak" after a few months and my boob just wouldn't get any larger no matter how much more or how much longer I used the cream. It didn't make the breast so large that it ever matched up to my right breast, it didn't even change my cup size, so I don't think it's worth it really.
I would say the only real way to increase your bust size is by gaining weight period. You have to lose weight ALL OVER to lose weight, and it's basically works the same way for your breasts. You can't gain some fat in only one area without gaining everywhere, but naturally, your best option is to gain weight if you want larger breasts because it's the only sure-fire way to get bigger anywhere really.
I've heard all kinds of tales such as eating Rolaids once a day for a month will add inches to your chest -- not true. Believe me, I tried them all because for most of my life I've been humilliated by my two different sized breasts, even to the point of scheduling a consultation with a surgeon and saving the money to have it done. I came SOO close to having surgery on my left breast, and I'm so glad I had the change of heart only months before my appointment! It was a blessing in disguise because these days, I'd never have surgery done.
So really, it's a hard one because your options are limited unless you go so far as to have a surgery and you never know if you'll end up regretting that usually.