Assuming you are doing roped climbing and are tied into the rope – a mistake I’ve made more than once – here’s a descriptive article on belaying.
The five most important, ranked parameters that decide the softness of the fall are;
1. The belaying technique: Moving inwards can prolong the fall by meters but if you instinctively sit-down you can instead reduce the fall by some -20 cm.
2. Rope condition: A worn out rope can easily reduce the softness by a meter or so.
3. Rope drag: A fall, especially high up, gets harder by the level of the rope drag.
4. Belay device: The ATC may increase the softness by 20 cm.
5. Weight: Lighter climbers will get harder and more static falls.
So you want a heavy climber and a light belayer using an ATC on a newer rope with minimum drag. (Although, as the article points out, the actual belay device plays a minor role.) So, those of you lighter lead climbers out there – bulk up. Those of you heavier belayers out there – start leading.