Comparison between the dust-free cavity sizes as revealed with mid-IR interferometry of the six post-AGB binary systems hosting transition discs in our sample and transition discs around YSOs from Varga et al. (2018) as a function of stellar luminosity. The sizes are displayed as either the physical size (top) or as the inner rim size relative to the dust sublimation radius (bottom). The latter is independent of the distance uncertainties. The sample of full discs of both the YSOs and post-AGB discs are overlaid in light grey (from Varga et al. 2018) and dark grey (this work).