Why should the AK be weaker than something that fires 5.56?
Weapon characteristics and bullet dynamics. The 5.56x45 was designed for maximum lethality at distances up to and exceeding 300 meters. The 7.62x39 was developed as a "drop" bullet. At closer ranges the 7.62 has more kinetic energy and thus more power, but it's a bull, it just rushes through cavities and leaves little more than a large hole, typically in and out, unless hitting bone wherein it would shatter the bone.
The 5.56 is designed to boat-tail and cavitate. When the bullet hits flesh it rolls over itself, end-over-end, as you can imagine, this causes massive internal injuries. Also, it is much much faster than the 7.62 so it carries more kinetic energy at distance. The 5.56 is also notorious for two things, shattering into several razor-sharp particles when it hits larger bones at shorter distances, causing major trauma, and "crawling-up" smaller bones at medium ranges. When the bullet "crawls-up" these bones, it pierces the front side of the bone and hits the far side at an angle which makes it travel along the marrow way inside. When it reaches the end of the bone it either mushrooms and blows out the back side of the victim or it again shatters into several pieces.