We employ fast scanning calorimetry (FSC) to investigate how varying the molecular weight interferes in the vitrification kinetics and molecular mobility of micrometric poly(4-tert-butyl styrene) (PtBS). In line with recent results, we find marked negative deviations of the fictive temperature T-f that is, the metric employed to characterize vitrification kinetics, with the sample size and for all molecular weights. In contrast, the molecular mobility remains size independent and bulk-like. This implies that vitrification kinetics and molecular mobility are decoupled. Furthermore, quantifying the magnitude of such decoupling through the Frenkel-Kobeko-Reiner approach, indicates that this is more pronounced in the high molecular weight samples than in the low molecular weight PtBS employed in the present work.