  Many of the powerful radio galaxies observed at high redshift are very small,
presumably because they are very young. A simple model, which treats the radio
source as a bubble expanding into a radially stratified medium, is capable of
explaining the main features of their size and luminosity evolution. In
particular, the model predicts strong negative luminosity evolution with
increasing size, thus accounting for the large number of short-lived small
sources in flux-limited samples.
  A variety of observational characteristics can be understood within the
framework of the bubble model. I address three of these: 1) the number versus
size statistics, which may provide a clue that jets in radio galaxies are
intermittent, with "on-off" cycles lasting only ten to one hundred thousand yr;
2) the rarity of spectral steepening due to synchrotron cooling at frequencies
of a few GHz, and its implications for source geometry and assumptions about
equipartition; and 3) the prevalence of gigahertz-peaked spectra among small
sources.
  I argue that all of these phenomena are primarily the consequence of the
youth and compactness of the radio source, and depend only weakly on
environment. There is no need to postulate an ISM with properties any different
from that of an ordinary nearby galaxy.
