  We use the Hubble Ultra Deep Field to study the galaxy luminosity-size (M-Re)
distribution. With a careful analysis of selection effects due to both
detection completeness and measurement reliability we identify bias-free
regions in the M-Re plane for a series of volume-limited samples. By comparison
to a nearby survey also having well defined selection limits, namely the
Millennium Galaxy Catalogue, we present clear evidence for evolution in surface
brightness since z ~ 0.7. Specifically, we demonstrate that the mean,
rest-frame B-band surface brightness for galaxies in a sample spanning 8
magnitudes in luminosity between M = -22 and -14 mag increases by ~1.0
mag/arcsec^2 from z ~ 0.1 to z ~ 0.7. We also highlight the importance of
considering surface brightness dependent measurement biases in addition to
incompleteness biases. In particular, the increasing, systematic
under-estimation of Kron fluxes towards low surface brightnesses may cause
diffuse, yet luminous, systems to be mistaken for faint, compact objects.
