  A sample of very high resolution cosmological disk galaxy simulations is used
to investigate the evolution of galaxy disk sizes back to redshift 1 within the
Lambda CDM cosmology. Artificial images in the rest frame B band are generated,
allowing for a measurement of disk scale lengths using surface brightness
profiles as observations would, and avoiding any assumption that light must
follow mass as previous models have assumed. We demonstrate that these
simulated disks are an excellent match to the observed magnitude - size
relation for both local disks, and for disks at z=1 in the magnitude/mass range
of overlap. We disentangle the evolution seen in the population as a whole from
the evolution of individual disk galaxies. In agreement with observations, our
simulated disks undergo roughly 1.5 magnitudes/arcsec^2 of surface brightness
dimming since z=1. We find evidence that evolution in the magnitude - size
plane varies by mass, such that galaxies with M* > 10^9 M_sun undergo more
evolution in size than luminosity, while dwarf galaxies tend to evolve
potentially more in luminosity. The disks grow in such a way as to stay on
roughly the same stellar mass - size relation with time. Finally, due to an
evolving stellar mass - SFR relation, a galaxy at a given stellar mass (or
size) at z=1 will reside in a more massive halo and have a higher SFR, and thus
a higher luminosity, than a counterpart of the same stellar mass at z=0.
