What effect does breast fat have on coopers droop?

Weight not fat: Cooper's droop is an expression referring to breast sag. The cooper's ligaments hold and support the breast. Excessive weight on the ligaments will over time cause stretching and sag. Whether the weight comes from fat or engorged milk glands it is the unsupported load on the ligaments that cause them to stretch over time. The solution is to wear a properly fitted and supportive bra every day.