Posterior predicted mass-size and fundamental plane relation. Top: distribution in stellar mass and half-light radius of the parent population (magenta regions) and the SLACS lenses (dashed contours), marginalised over the model parameters η. The dotted black line is the mass-size relation from Hyde & Bernardi (2009). The solid blue line indicates the average size of SLACS lenses as a function of stellar mass. Bottom: distribution in velocity dispersion, as a function of the predicted average given the stellar mass and size (the mean of the distribution in Equation (36)). The solid blue line indicates the average velocity dispersion of the SLACS lenses.