Ultrafast electron diffraction is a sensitive tool to directly study molecular dynamics in structural detail. Here, we report the influence of counterion size on the photoinduced insulator-to-metal phase transition in two derivatives of the organic salt (EDO-TTF)2XF6. For X = P, three dominant motions are present and the molecules undergo the transition, whereas in the case of X = Sb, only two dominant motions are found and the molecules do not evolve into the metallic state. This reduction in dimensionality is supported by a novel data analysis method involving singular value decomposition of time-resolved electron diffraction data in reciprocal space.