Could quasars' quick brightness chambers result from the same process everywhere rather than small size?

Why is a rapid change in luminosity seen as implying a quasar is small? If there was a cloud made of a radioactive isotope with a half life of a few seconds several light years across, it would still change rapidly without its constituents interacting. Why is the same rate of change in a quasar taken as entailing it's small or due to relativistic jets?