This goes back to elastic v plastic deformation. Proper tightening should result in elastic deformation of parts, where any change in dimension is completely reversed when the part is removed or the bolts de-tensioned. If you go beyond the range of deformation that is elastic, you get irreversible plastic deformation, which actually represents weakening of the crystal structure of the materials, in this case the aluminum of the base of the crank arm. With enough of this over tightening, the crank arm will fracture, possibly during a climb.