Making sure your bra fits correctly can improve your health. Wearing the wrong size and suffering from the discomfort and pain for a long period of time can cause permanent health issues. Bad bras can cause bad posture, damaging the muscles in the chest and back. If it doesn’t provide enough support, it can cause pain in the breasts or chafe skin.
Million Looks has more information. http://www.millionlooks.com/shopwindow/the-harm-of-a-poorly-fitting-bra/