Different sports. Pereira can finish guys once he knocks them down, boxers have to wait through a 10 count. And Periera usually finishes guys after he hurts them. He doesn´t actually just one shot them, at least not the majority of the guys he has stopped. Imagine if Foreman were allowed to wail on his downed opponents. Oh lawd!
Not denying that, just saying that the MMA comparison isn´t fair and he usually finishes guys with followup shots instead of a once shot KO. He clearly has a lot of power, but let´s not get carried away here.