More load _reduces_ the ripple? That's very
unusual.
Perhaps a parasitic oscillation that's defeated by more load
because now the loop gain is below 1?
But loading the +5V output also reduces the ripple on the +12V
output. Loading the +5V output would, I think, vary the mark-space
ratio of the drive to the main chopper and thus the drive signal to
the 555 in the +12V regulator. Not that I can see how that would
affect things.
-tony