For a binary liquid mixture of hard discs well above the freezing density o
f the one-component liquid the size dependence of the self-diffusion coeffi
cients of small and large particles is calculated by molecular dynamics sim
ulation. The observed size effect can be satisfactorily explained by the si
ze-dependent cut-off of long-wavelength fluctuations of transverse collecti
ve flow and gives no indication of the importance of more 'cooperative' pro
cesses at high densities.