  Planets with 2 $R_{\oplus}$ < $R$ < 3 $R_{\oplus}$ and orbital period $<$100
d are abundant; these sub-Neptune exoplanets are not well understood. For
example, $Kepler$ sub-Neptunes are likely to have deep magma oceans in contact
with their atmospheres, but little is known about the effect of the magma on
the atmosphere. Here we study this effect using a basic model, assuming that
volatiles equilibrate with magma at $T$ $\sim$ 3000 K. For our Fe-Mg-Si-O-H
model system, we find that chemical reactions between the magma and the
atmosphere and dissolution of volatiles into the magma are both important.
Thus, magma matters. For H, most moles go into the magma, so the mass target
for both H$_2$ accretion and H$_2$ loss models is weightier than is usually
assumed. The known span of magma oxidation states can produce sub-Neptunes that
have identical radius but with total volatile masses varying by 20-fold. Thus,
planet radius is a proxy for atmospheric composition but not for total volatile
content. This redox diversity degeneracy can be broken by measurements of
atmosphere mean molecular weight. We emphasise H$_2$ supply by nebula gas, but
also consider solid-derived H$_2$O. We find that adding H$_2$O to Fe probably
cannot make enough H$_2$ to explain sub-Neptune radii because $>$10$^3$-km
thick outgassed atmospheres have high mean molecular weight. The hypothesis of
magma-atmosphere equilibration links observables such as atmosphere
H$_2$O/H$_2$ ratio to magma FeO content and planet formation processes. Our
model's accuracy is limited by the lack of experiments (lab and/or numerical)
that are specific to sub-Neptunes; we advocate for such experiments.
