If a standard lump on the ground is a
molehill, what makes it a mountain?

Cereoid and Pieter.

What about pawpaw (_Carica papaya_) "trees" which are really very large
herbs?

Pawpaw is Asimina triloba, a member of the Annonaceae, definitely a tree.
Carica papaya, the papaya, is referred to as a plant, not a tree, same as a
banana plant.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)