  Extensive molecular dynamics (MD) computer simulations of an equimolar
glass-forming AB mixture with large size ratio are presented. While the large A
particles show a glass transition around the critical density of mode coupling
theory $\rho_c$, the small B particles remain mobile with a relatively weak
decrease of their self diffusion coefficient $D_{\rm B}$ with increasing
density. Surprisingly, around $\rho_c$, the self-diffusion coefficient of the A
particles, $D_{\rm A}$, also starts to follow a rather weak dependence on
density. We show that this is due to finite-size effects that can be understood
from the analysis of the collective interdiffusion dynamics.
