What is a Substantial Word™?

This is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.

If a word conforms to a special rule, I call it a Substantial Word™.
Use the examples below to find the rule.

Note: there is a rule for being on the Substantial list and a rule for being on the Insubstantial list. You must find both.

$$\begin{gather*} % set Title text. (spaces around the text ARE important; do not remove.) % increase Pad value only if your entries are longer than the title bar. % \def\Pad{\P{0.0}} % \def\S#1#2{\Space{#1}{20px}{#2px}}\def\P#1{\V{#1em}}\def\V#1{\S{#1}{9}} \def\T{\textbf{Substantial Words}^{\;\!™}\Pad}\def\NT{\Pad\textbf{Insubstantial Words}^{\;\!™}\Pad\ } \smash{\lower{29px}\bbox[lightblue]{\phantom{\rlap{rubio.2019.05.15}\S{6px}{0} \begin{array}{cc}\Pad\T&\NT\\\end{array}}}}\atop\def\V#1{\S{#1}{5}} \begin{array}{|c|c|}\hline\Pad\T&\NT\\\hline % \text{ dwindled}&\text{ terminology}\\ \hline \text{ flabbergasted}&\text{ marshmallows}\\ \hline \text{ tablecloth}&\text{ mathematician}\\ \hline \text{ abridged}&\text{ splittable}\\ \hline \text{ welcoming}&\text{ maintain}\\ \hline \text{ overcast}&\text{ diminutive}\\ \hline \text{ hamburger}&\text{ pseudonym}\\ \hline \text{ accomplishment}&\text{ psychotherapists}\\ \hline \text{ segregation}&\text{ nowhere}\\ \hline \text{ breadboard}&\text{ probably}\\ \hline \hline \end{array} \end{gather*}$$

Many more Substantial and Insubstantial Words exist and are not listed above. Some English words are neither substantial nor insubstantial and some are both.

CSV version:

Substantial Words™, Insubstantial Words™
dwindled            terminology
flabbergasted,      marshmallows
tablecloth,         mathematician
abridged,           splittable
welcoming,          maintain
overcast,           diminutive
hamburger,          pseudonym
accomplishment,     psychotherapists
segregation,        nowhere


What is the special rule these words conform to?

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, pseudonym
accomplishment, psychotherapists
segregation, nowhere