I want to lose weight but my breasts are a size 34a-34b. And if i lose weight, i'm afraid they're gonna be too small and i won't be able to even fit in any size bra! So i just wanna know how much of my weight loss will affect my breasts.

When losing weight, fat loss can not be targeted to a specific area. It is evenly burned throughout the body. However, if your breast are already small, they probably don't contain many fat cells and the change in size will probably be minimal. It also depends on how many pounds you are wanting to lose. Five pounds or 50 pounds will have totally different results.

From my health education which was brief plus what I saw on eductational programming, the breasts are only a fraction of fat distribution in women often (and this is more for obesity) women tend to take on a gynoid or pear shape as they gain weight. most of the fat is in the lower stomach, hips, and brace yourself, butt. with the remaining portion going to the chest area I can't give you more than that without the possibility of being wrong

It is not going to be a proportional loss all over the body parts. So the breast would not be affected in the same ratio but less.

For most folks, one loses weight in the last place you added it. It also depends on your body type. If you haven't gained bra sizes as you've put on weight, then it's unlikely you'll shrink them. However, if you've got some additional fat deposits (actually enlarged fat cells, fat cells don't appear and disappear) on you back, then you'll notice that your bras might fit a little looser.
Good luck with your plan, and stick with it. It took time to put the weight on, it'll take time to remove it. (3500 calories = 1lb of body fat)

depends on how much weight u lose more than a stone u will proberley go down a size