I am a 5'2'' female, in the past year I have gained some weight, and also gone onto birth control so my breasts have gotten considerably larger. It is now to the point where I have a ton of stretch marks on my breasts (like a TON) and its even painful at times. Is this normal, and what can I do to make it better? Going off birth control is not an option and I am already on a low estrogen dose.
Your birth control pills contain a progestin component as well as the estrogen. As you noted, you would likely do well to not increase the estrogen dose, but you could consider a pill switch to a different progestin, to see if that makes a difference. Genetics and your initial/current BMI are certainly factors with the symptoms you describe.
It is usually fairly straightforward for your provider to switch from one pill to another, if you choose to give that a try.