36DD here and totally happy with it. I want to lose a few pounds, but have been resisting because I actually don't want to lose any breast volume!
Also, I noticed a theme reading through this thread - a lot of girls with cup sizes on the small to average (A,B,C) complaining about back pain. I'm going to offer some advice that might not seem good, but I promise it will help.
If you are wearing a B or C cup and have back pain you need to get a fitting a go UP in cup size and DOWN in band size. You breasts need to receive their support from underneath, which mean you need a snugly fitting band. And
that means that the first few times you wear your bra that band will feel uncomfortably tight, because it has to be in order to still be supportive once it stretches and has been washed. If you have a band that is too loose, you will be relying on your straps for support, and that is putting pressure on your shoulders and back, causing pain.
With the decrease in bad sizing comes the increase in cup sizing (known as sister sizes). Which means that a 34C may actually need a 32D to feel comfortable. Or, even, a 30DD if she is very petite. Obviously, this is a size difficult to find which is why unscrupulous retailers (*cough Victoria's Secret cough*) will sell you the "next best thing" that is actually hurting your body.
With the larger cup sizes (DDD/E,F,G,H) the pain is a matter of sheer volume, and I am sure you ladies are already well aware of the limited number of supportive options that you have.
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My best suggestion for women of ALL cup sizes who are having difficulties and discomfort with their bras is to get properly fitted and then order one or two bras in your correct size. Bras shouldn't be washed after every wear (goes with the same diminishing support in the band that I mentioned above) so you don't need one in every color. A white and a nude, or perhaps nude and black is enough to get started.
If you don't trust the gals at the local mall, there are companies on line that will tell you what measurements to take, and then you can send them in, along with any concerns, and they will help you find your size. They usually are the ones who offer the greatest size selection.
If anyone ever has any questions, I am always free to answer them. I managed a lingerie shop for three years, and have seen just about every problem under the sun.
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ETA: If your pain stems from something that can't be aided with a better-fitting bra, I am so sorry because that is awful. If you've tried different sizes and nothing works, I definitely agree with the above advice and see a medical professional who can rule out anything internal causing pain.