This paper compares the shrinkage during pyrolysis of a gel precursor as thin film and as bulk sample. The hybrid silica gel, precursor for SiOC glasses, contains Si–CH3 and Si–H moieties. The shrinkage of bulk samples has been measured with conventional dilatometry. Shrinkage of thin films has been studied for the first time with in situ dilatometry allowing to measure the thickness and the refractive index during pyrolysis. Thin films shrink more compared to bulk samples and the pyrolytic transformation occurs at lower temperature (100–150 °C) compared to the bulk samples.