  We investigate and compare the composition of M-dwarf planets in systems with
only one known planet (``singles") to those residing in multi-planet systems
(``multis") and the fundamental properties of their host stars. We restrict our
analysis to planets with directly measured masses and radii, which comprise a
total of 70 planets: 30 singles and 40 multis in 19 systems. We compare the
bulk densities for the full sample, which includes planets ranging in size from
$0.52 R_{\oplus}$ to $12.8R_\oplus$, and find that single planets have
significantly lower densities on average than multis, which we cannot attribute
to selection biases. We compare the bulk densities normalized by an Earth model
for planets with $R_{p} < 6R_{\oplus}$, and find that multis are also denser
with 99\% confidence. We calculate and compare the core/water mass fractions
(CMF/WMF) of low-mass planets ($M_p <10 M_{\oplus}$), and find that the likely
rocky multis (with $R_p <1.6 R_{\oplus}$) have lower CMFs than singles. We also
compare the [Fe/H] metallicity and rotation period of all single versus
multi-planet host stars with such measurements in the literature and find that
multi-planet hosts are significantly more metal-poor than those hosting a
single planet. Moreover, we find that host star metallicity decreases with
increasing planet multiplicity. In contrast, we find only a modest difference
in the rotation period. The significant differences in planetary composition
and metallicity of the host stars point to different physical processes
governing the formation of single- and multi-planet systems in M dwarfs.
