Well certainly then because of reduced X radiation
from the CRT.
The smaller fonts disturb less phosphor.
But if the EHT regulation of your monitor is poor, then the EHT will
actually rise on a mostly dark screen, so the energy of the X-rays will
rise too.
(for the humor-impaired, this is a joke. If your EHT regulation was that
bad, you'd have terrible convergence and purity problems. Actually,
having seen some PC monitors, though....)
-tony