A bag that fits well stops the board from moving inside and getting damaged. Therefore, you get a better fit and more protection. Keep in mind that manufacturers give a little bit of space inside the bag. If you get a bag that is too small, then the board moves and can be damaged as it bumps against hard objects. They may shift when you are transporting them on your back, which can cause you to lose your balance, and run the risk of damaging the board, too.