@breizh they specifically said that it wasn't about his strength, but about his weight. He just doesn't have the mass to do damage with a kick, no matter how fast a bullet is fired it has limits to what it can penetrate.
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.
@breizh Applying great force requires a contact point with something that doesn't move - like the ground in this case. Him jumping and then hitting them results in just an impulse of m * v, where m is his mass and v is his velocity. Clearly his mass is too small comparatively. Note that he used their mass against them in both cases.