  Gas-phase abundances and abundance gradients provide much information on past
stellar generations, and are powerful probes of how galaxies evolve. Gas
abundance gradients in galaxies have been studied as functions of galaxies'
mass and size individually, but have largely not been considered across the
galaxy mass--size plane. Thus, we investigate gas-phase abundance gradients
across this plane, using a sample of over 1000 galaxies selected from the
MApping Nearby Galaxies at APO (MaNGA) spectroscopic survey. We find that
gradients vary systematically such that above $10^{10}M_{\odot}$, smaller
galaxies display flatter gradients than larger galaxies at a given stellar
mass. This mass--size behaviour cannot be explained by instrumental effects,
nor is it simply a reflection of known trends between gradients and morphology.
We explore multiple possibilities for a physical origin for this pattern,
though further work is needed to establish a firm physical interpretation.
