  Low-metallicity dwarf galaxies often show no or little CO emission, despite
the intense star formation observed in local samples. Both simulations and
resolved observations indicate that molecular gas in low-metallicity galaxies
may reside in small dense clumps, surrounded by a substantial amount of more
diffuse gas, not traced by CO. Constraining the relative importance of
CO-bright versus CO-dark H2 star-forming reservoirs is crucial to understand
how star formation proceeds at low metallicity. We put to the test classically
used single component radiative transfer models and compare their results to
those obtained assuming an increasingly complex structure of the interstellar
gas, mimicking an inhomogeneous distribution of clouds with various physical
properties. We compute representative models of the interstellar medium as
combinations of several gas components, each with a specific set of physical
parameters. We introduce physically-motivated models assuming power-law
distributions for the density, ionization parameter, and the depth of molecular
clouds. We confirm the presence of a predominantly CO-dark molecular reservoir
in low-metallicity galaxies. The predicted total H2 mass is best traced by [C
II]158um and, to a lesser extent, by [CI] 609um, rather than by CO(1-0). We
examine the CO-to-H2 conversion factor vs. metallicity relation and find that
its dispersion increases significantly when different geometries of the gas are
considered. We define a clumpiness parameter that anti-correlates with [CII]/CO
and explains the dispersion of the CO-to-H2 conversion factor vs. metallicity
relation. We find that low-metallicity galaxies with high clumpiness may have
CO-to-H2 conversion factor as low as the Galactic value. We identify the
clumpiness of molecular gas as a key parameter to understand variations of
geometry-sensitive quantities, such as CO-to-H2 conversion factor.
