This is incorrect. The engines are not too powerful for the airframe. The problem is that the engines themselves create lift at high Angles of Attack, pitching the nose of the plane up.

"This new location and size of the nacelle causes it to produce lift at high AoA; as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the aircraft closer towards the stall."

http://www.b737.org.uk/mcas.htm