  The phase diagram of a binary fluid mixture of highly asymmetric additive
hard spheres is investigated. Demixing is analyzed from the exact low-density
expansions of the thermodynamic properties of the mixture and compared with the
fluid-fluid separation based on the effective one-component description.
Differences in the results obtained from both approaches, which have been
claimed to be equivalent, are pointed out and their possible origin is
discussed. It is argued that to deal with these differences new theoretical
approximations should be devised.
