The biggest reason why you should wear a proper-fitting sports bra is comfort. A runner should never think about her breasts while running. This is a key indication that the sports bra isn't fitting properly. A poor-fitting sports bra can hinder performance and ruin your entire workout experience. If the sports bra is too tight, it can result in chafing and prevent proper breathing. A sports bra that is too loose won't provide the right amount of support to reduce breast movement. Try on a sports bra as you would running shoes. While testing out the sports bra, you shouldn't feel bounce or pressure against your chest wall.
First you need to find out what size sports bra you actually wear by going through a professional fitting.
Straps : They shouldn’t be too tight or dig into your shoulders. If the straps are providing a majority of the support, the sports bra isn’t the right fit. Adjustable straps provide a customizable fit.
Band : As the foundation of the bra, the band should fit against your ribcage. Take a deep breath to make sure the bra fits comfortably. Look for a wide band with strong elastic to provide necessary support.
Cups : Your breasts should be fully covered and should not be pouring out of the top. The fabric should be smooth with no wrinkles.