@breizh
Because punch relies not just on your strength, but also how much mass is behind your punch. You can be super strong and whatever, but if your opponent is ten times larger then you'd need to match his size (or close to it) to do damage.
Imagine throwing a rock at an elephant. If that rock is 5kg, but the elephant is 1000 times heavier, he won't feel much - compare that to throwing a charging elephant.
And Purchinov has that whole blessing of the earth that he can use to increase his physical stats, letting him throw creatures of that size.