This is why, for example, using bustards as a metric of near maximum sizes
for volancy in birds is problematic.

--------This is true. The Whooper Swan 'Stonker' (JAP) was as massive as a
Kori Bustard, flew by means of continous flapping, and traveled regularly
between Scotland, Iceland, and sometimes Ireland. He was a flier of the
first order, and doesn't seem to have been struggling to achieve volancy.
Once, during a gale off the coast of Iceland, while wearing a radio tracker,
he made one of the most remarkable emergency endurance flights that I am
aware of.

I agree this may be true; I also think such conditions may be more
widespread than we give credit for.

-------Yes. I tend to think it is a fluke that there aren't more giant
fliers at the moment.

It appears that pterosaurs ended up huge more regularly than birds (and
became much large-bodied overall) because their membranous wings and
launch system made large size especially viable.

------Yes, though I note that for an equivilent span, pterosaurs tended to
have smaller torsos than birds -- but their heads and necks tended to be
larger, sometimes much larger.