See a physical therapist for details. But the basics are: Learn good posture. Often those with large breasts hunch over to hide them and that causes the pain much more than their weight does. Correct posture is head back, shoulders back, chest forward (yes boobs sticking way out), stomach straight. Even very large breasts only weigh a couple pounds. Unless they’re literally the size of bowling balls no that’s not the reason. Either way do back exercises such as pull-ups or seated rows, because the rest of your torso does weigh about 40 pounds. Google how to correctly measure bra size so you can get a properly fitting bra. Often cup is too low and band is too high due to old myths. Don’t be surprised if you get some high letter like K, it’s not actually that large.
Many women go to YA asking about a reduction, as if it’s a magic wand. That’s usually not the cause per above and then insurance won’t pay for it. It is also extreme surgery with risks and difficult recovery.