  Although dust content is usually assumed to depend uniquely on metallicity,
recent observations of two extremely metal-poor dwarf galaxies have suggested
that this may not always be true. At a similar oxygen abundance of ~ 3% Zsun,
the dust-to-gas and dust-to-stellar mass ratios in SBS 0335-052 and IZw 18
differ by a factor 40-70 according to including molecular gas or excluding it.
Here we investigate a possible reason for this dramatic difference through
models based on a semi-analytical formulation of chemical evolution including
dust. Results suggest that the greater dust mass in SBS 0335-052 is due to the
more efficient grain growth allowed by the high density in the cold
interstellar medium (ISM), observationally inferred to be almost 20 times
higher than in IZw 18. Our models are able to explain the difference in dust
masses, suggesting that efficient dust formation and dust content in galaxies,
including those with the highest measured redshifts, depend sensitively on the
ISM density, rather than only on metallicity.
