This model was wearing a 36 band in this shot.
Notice the upward arch in the back of this bra. The roominess of the band is allowing the weight of the breasts to pull the back of the bra up.
Your breasts will never get the support they need if your band is too loose. Period.
80% of the support and lift of the breast comes from the underband. You should never adjust the straps tighter to lift your breast if you don't have a proper fitting band size. It will never work, and physics suggest that if your band is too loose and your straps are cranked as tight as you can make them......YOUR BOOBS WILL HEAD SOUTH!
~Megan Summerville