The plane isn’t perfectly rigid, so it would only be able to impart as much energy to the structure as contained in the part of the plane that gets sheared off while colliding with the containment building.

As a layman there are probably few theories behind this statement: 1. The plane is not an energy monolith, any part of the plane larger than the building (eg wings) would have its own inertia which continues to rip off and around the building rather than drive all energy/mass attached to plane to the point of impact. 2. A smaller building may act like a plow if circular or hit on a corner, diverting energy as in 1. 3. If you simplify the side of a building to a supported plane, a larger plane may deform to failure more than a small plane due to the distance between supports ( other walls of building)

My read is that the sensitive area of the reactor is a lot smaller than the plane, so only part of the energy of the plane would hit it, the rest would go into the ground around it or whatever else the plane hits.

I’m no expert at physics but the towers were more “collapsible” so they would absorb more of the momentum from the planes.