  Analyses of strong gravitational lenses, galaxy-scale kinematics, and
absorption line stellar population synthesis (SPS) have all concluded that the
stellar initial mass function (IMF) varies within the massive early-type galaxy
(ETG) population. However, the physical mechanism that drives variation in the
IMF is an outstanding question. Here we use new SPS models to consider a
diverse set of compact, low-velocity dispersion stellar systems: globular
clusters (GCs), an ultra-compact dwarf (UCD), and the compact elliptical (cE)
galaxy M32. We compare our results to massive ETGs and available dynamical
measurements. We find that the GCs have stellar mass-to-light ratios (M/L) that
are either consistent with a Kroupa IMF or are slightly bottom-light while the
UCD and cE have mildly elevated M/L. The separation in derived IMFs for systems
with similar metallicities and abundance patterns indicates that our SPS models
can distinguish abundance and IMF effects. Variation among the sample in this
paper is only $\sim 50\%$ in normalized M/L compared to the $\sim 4\times$
among the ETG sample. This suggests that metallicity is not the sole driver of
IMF variability and additional parameters need to be considered.
