I don't know from where you got the 50% more weight figure of 7.62 NATO round compared with 7.62 AK round.
The bulk production NATO round (M80) weighs 147 gr, while the AK round (M43) weighs 123 gr. Thus the M80 weighs just about 19.51% more compare to the M43.
A
7.62×51 M80 cartridge has a 147 grain bullet loaded to a muzzle velocity of 2,800 fps. This gives it a muzzle energy of 2,559 ft lbs. The 7.62×39 M43 cartridge often has a 123 grain bullet loaded to a muzzle velocity of about 2,350 fps. This gives it a substantially lower muzzle energy of 1,508 ft lbs. That makes the 7.62×51 about 40 percent more powerful in terms of kinetic energy.