H i size versus stellar mass relation. In the NIHAO simulations (red points) sizes enclose 90 per cent of the H i mass from a face-on projection. In the SPARC observations (blue points) sizes are the last point on the H i rotation curve. Points show individual galaxies, circles show mean sizes in bins of H i mass, the error bar shows the error on the mean, and solid and dashed lines show a linear fit and its standard deviation.