To be honest I think it is down to your skins elasticity,skin thins over time,and anything heavier than what you had previously,will stretch the skin,natural breasts would sag if not properly supported.xxx
I would imagine any implant will sag a bit more than your natural boobs would because they’re heavier like tina said. But I wouldn’t have thought there would be much of a difference in sagging between unders and overs, not enough for you to choose one placement over the other xx