This isn’t quite right, elephants do have bigger cells as well, so 1kg of elephant has fewer cells than 1kg of mice. There is also the fact that metabolism show

This isn’t quite right, elephants do have bigger cells as well, so 1kg of elephant has fewer cells than 1kg of mice. There is also the fact that metabolism shows sub linear scaling with body mass. Geoffrey West has various papers about this. As well as all that, elephant cells are intrinsically more cancer resistant as you point out.