Back and neck problems is common with smaller girls with larger breasts. A reduction might be in order if this becomes very problematic, although I do believe you're still too young to make this decision indefinately. I would be much more inclined to recommend you strengthen your back muscles, neck muscles, and tone your whole body while you're at it. In other words, go to the gym. There is no such thing as "breasts are too big for my back to support it"... It is more in the realm of, "my back is not strong enough to support my breasts".
Unless there are more severe side effects/problems from your breast size, I would say keep it as is until you're a bit older.
Justin