back pain is a myth and those who has it probably never had proper backs at all and walked like curved and never had proper exercises.
Actually, if you go back and search in the posts, Zorlun said long ago Myriam is inspired by a real friend with whom in the past they had even more than a friendship, and who really had big "characteristics" but relatively slim otherwise, and she ended up having to do a reduction, because it was a problem.
The elements you are indicating, about bad posture and lack of exercise, are indeed elements that matter, men get back problems from that (especially if there is extra tension accumulated in the muscles) all the time.
But it is also true that getting a couple of kilos off axis that pull you down constantly (and even more, they are a distributed weight that wobbles around), imply a constant work of muscles to maintain the standing up posture, considering also that in order to be able to more easily move, in reality we are not perfectly balanced and centered.