  The connection between dark matter halos and galactic baryons is often not
well-constrained nor well-resolved in cosmological hydrodynamical simulations.
Thus, Halo Occupation Distribution (HOD) models that assign galaxies to halos
based on halo mass are frequently used to interpret clustering observations,
even though it is well-known that the assembly history of dark matter halos is
related to their clustering. In this paper we use high-resolution
hydrodynamical cosmological simulations to compare the halo and stellar mass
growth of galaxies in a large-scale overdensity to those in a large-scale
underdensity (on scales of about 20 Mpc). The simulation reproduces assembly
bias, that halos have earlier formation times in overdense environments than in
underdense regions. We find that the stellar mass to halo mass ratio is larger
in overdense regions in central galaxies residing in halos with masses between
10$^{11}$-10$^{12.9}$ M$_{\odot}$. When we force the local density (within 2
Mpc) at z=0 to be the same for galaxies in the large-scale over- and
underdensities, we find the same results. We posit that this difference can be
explained by a combination of earlier formation times, more interactions at
early times with neighbors, and more filaments feeding galaxies in overdense
regions. This result puts the standard practice of assigning stellar mass to
halos based only on their mass, rather than considering their larger
environment, into question.
