We use large scale molecular dynamics simulations of the isomerizations of azobenzene molecules diluted inside a simple molecular material, to investigate the effect of a modification of the cis isomer shape on the induced diffusion mechanism. To this end we simulate incomplete isomerizations, modifying the amplitude of the trans to cis isomerization. We find thresholds in the evolution of the host molecules mobility with the isomerization amplitude, a result predicted by the cage- breaking mechanism hypothesis (Teboul, V.; Saiddine, M.; Nunzi, J.M.; Accary, J.B. J. Chem. Phys. 2011, 134, 114517) and by the gradient pressure mechanism theory (Barrett, C.J.; Rochon, P.L.; Natansohn, A.L. J. Chem. Phys. 1998, 109, 1505-1516. ). Above the threshold the diffusion then increases linearly with the variation of the chromophore size induced by the isomerization.