I'm not sure per kg energy scales like that. It would if the only thing that mattered was the energy dumped into the target - but it isn't. The depth of penetration, the proportion of vital organ damaged and the speed of blood loss are just as (or possibly more) important.
A 30 calibre hole in most animals below the size of a cow will kill them inside a few minutes. A .20 calibre sized hole will take much, much longer.
But if I shot a roe deer with a .20cal through say the lungs and you shot a red stag through the lungs with a .243 1mm difference in hole size, which would die first?