It's the firecracker principle.
You hold a firecracker in an open palm. It explodes. You suffer from burns. It's painful, but not that serious. At most, you may need stitches.
Close your fist around that same firecracker, though, and you can kiss your hand goodbye when it goes boom.
Why?
Explosions are omnidirectional and get progressively weaker as they propogate. In an open palm, most of the explosive force hits empty air. In a closed palm, nearly all of the explosive potential hits the hand, and the force is at (or very near) its maximum potential.
Same principle applies to the suits. He probably could have placed ten times the amount of explosives on the exterior, and all it would have done would be to blow the armor through the wall and maybe scratch the paint. However, because the explosion started inside the suits and propogated outwards from there, the suit encountered huge amounts of explosive force on its reletively unprotected interior.
Think of it like an Abrams tank. An Abrams can run over an IED and probably the worst thing they have to worry about is a thrown track. However, you pop the hatch and drop a single fragmentation grenade inside, and the tank can be considered mission-killed.
