Food safety authorities have already allowed the use of mathematical models to predict diffusion from plastic food contact materials. These models use the molecular weight of the migrant as a cornerstone parameter that describes the contribution of the migrant to the diffusion process. In this work, the dependence of the diffusion coefficient on the migrant size was examined through fluorescence recovery after photobleaching (FRAP). A model migrant series of fluorescent probes was used, covering a wide molecular weight range. The advantage and originality of the tested migrant series are associated with the fact that the same shape and chemical functionality are maintained regardless of the molecular weight of the migrants. In this way the dependence of the acquired data on parameters other than size is excluded. The same experiments were carried out in dry and hydrated polyamide 6 to evaluate the effect of polymer matrix mobility in the "diffusion-migrant size" relationship. The experimental data were compared to well-known mathematical or semiempirical approaches, verifying that there is a relationship between the diffusion coefficient and the size of the migrant. However, it is demonstrated that this relationship is also affected by the mobility of the polymer matrix, becoming more pronounced as the mobility of the matrix decreases.