In this work we study the effects of the optical shrinkage in polymer and liquid crystal (LC) mixtures optimized for their use as active media in compact plastic laser devices. These mixtures are characterized by the presence of the rhodamine 6G as an active dye. Modifications in the reflection properties of the gratings as a function of the active dye concentration have been determined experimentally and a detailed theoretical simulation of the optical transmittance properties of these devices is provided. Moreover, the comparison between two different experimental approaches clarifies the contribution to the optical shrinkage due to the presence of the active dye. In principle this approach allows determining the linear mechanical shrinkage by separating the contribution to optical shrinkage due to photochemical transformations from that due to mechanical effects.