  In the past, the cumulative metallicity distribution function (CMDF) turned
out as a useful tool to constrain the accretion history of various components
of the Milky Way. In this Letter, by means of analytical, leaky-box chemical
evolution models (i.e. including both infall and galactic outflows) we study
the CMDF of local star-forming galaxies that follow two fundamental empirical
scaling relations, namely the mass-metallicity and main sequence relations. Our
analysis shows that galactic winds, which are dominant mostly in low-mass
systems, play a fundamental role in shaping this function and, in particular,
in determining its steepness and curvature. We show that the CMDF of low-mass
(M$_{\star}$/M$_{\odot} \le 10^{9.5}$) and high-mass
(M$_{\star}$/M$_{\odot}$>10$^{10.5}$) galaxies deviate substantially from the
results of a 'closed-box' model, as the evolution of the former (latter)
systems is mostly dominated by outflows (infall). In the context of galactic
downsizing, we show that downward-concave CMDFs (associated with systems with
extremely small infall timescales and with very strong winds) are more frequent
in low-mass galaxies, which include larger fractions of young systems and
present more substantial deviations from equilibrium between gas accretion and
reprocessing (either via star formation or winds).
