We investigate simultaneous lamellar contraction and the stress-strain response in styrene-ethylene propylene (SEP) lamellar diblock copolymer melts under shear. Lamellar contraction was found in predominately parallel lamellae in three SEP copolymers during steady shear. The amount of contraction strongly depends on the molecular weight of the block copolymers as well as the rate of deformation. These molecular weight and shear rate dependences are discussed in terms of chain distortions and entanglements in the interpenetration zones of the block copolymer lamellae. For three molecular weights and three shear rates, we report a unifying correlation between lamellar contraction and applied shear stress, indicating a strong dependence of the molecular-scale chain distortion and the macroscopic strength of deformation (stress).