Substantial and Insubstantial words are
those whose letters contain, in order, a word meaning "large" or "small" respectively.
Thus:
dwindled terminology
flabbergasted, marshmallows
tablecloth, mathematician
abridged, splittable
welcoming, maintain
overcast, diminutive
hamburger, pseuddonym
accomplishment, psychotherapists
segregation, nowhere
breadboard, probably
Credit where due: Stiv
found PUNY and SHORT where I had the clearly-inferior PONY and CHEAP.