  Massive quiescent galaxies have much smaller physical sizes at high redshift
than today. The strong evolution of galaxy size may be caused by progenitor
bias, major and minor mergers, adiabatic expansion, and/or renewed star
formation, but it is difficult to test these theories observationally. Herein,
we select a sample of 35 massive, compact galaxies ($M_* = 1-3 \times 10^{11}$
M$_\odot$, $M_*/R^{1.5} > 10^{10.5}$ M$_\odot$/kpc$^{1.5}$) at $z=2$ in the
cosmological hydrodynamical simulation Illustris and trace them forward to
$z=0$ to uncover their evolution and identify their descendants. By $z=0$, the
original factor of 3 difference in stellar mass spreads to a factor of 20. The
dark matter halo masses similarly spread from a factor of 5 to 40. The
galaxies' evolutionary paths are diverse: about half acquire an ex-situ
envelope and are the core of a more massive descendant, a third survive
undisturbed and gain very little mass, 15% are consumed in a merger with a more
massive galaxy, and a small remainder are thoroughly mixed by major mergers.
The galaxies grow in size as well as mass, and only $\sim$10% remain compact by
$z=0$. The majority of the size growth is driven by the acquisition of ex-situ
mass. The most massive galaxies at $z=0$ are the most likely to have compact
progenitors, but this trend possesses significant dispersion which precludes a
direct linkage to compact galaxies at $z=2$. The compact galaxies' merger rates
are influenced by their $z=2$ environments, so that isolated or satellite
compact galaxies (which are protected from mergers) are the most likely to
survive to the present day.
