A rabbits head creates even less resistance than the wax so produces even less braking effect on a heavy pellet than the wax does, so the impact velocity and the amount of energy dumped is even more important, what the wax shows, unlike ballistic gel which is elastic so closes up, is the actual cavity the pellet creates, the bigger the cavity the greater the trauma that is caused, a good example was the .20 pellet which heavier than a .177 but only slightly larger in surface area it's core was extremely small, it took most of it's energy with it, and that's exactly what you don't want, ideally you want the hollowpoint or wadcutter, but as we all know few of these pellets will group at longer ranges.
But even then, because the energy levels are so low, because the rabbit your shooting has very little resistance to the pellet, shot placement is the critical function, and the flatter flying light pellet is more forgiving than the slower more looping trajectory of the heavy, when you then add in the pellet creating more impact damage and getting there faster the choice is clear.