In several major traditions the commentarial literature on a text runs to many times the length of the text itself. The imbalance follows from how these works were composed and used.
The source texts were compressed on purpose
Foundational works in aphoristic or verse form were built for memorisation, which required brevity. Explanation was supplied by a teacher rather than by the page.
A text of that kind is closer to an index than to an exposition. Its lines mark positions in an argument that a trained reader is expected to reconstruct.
Once the oral explanation thinned, the reconstruction had to be written down, and the written version is necessarily much longer than what it explains.
Later readers do not share the original context
Every generation reads with different assumptions and encounters different difficulties, so questions the first audience never asked become pressing later.
A commentary answers the questions of its own moment, which means a text acquires new commentaries even when adequate older ones exist.
Layers accumulate rather than replace one another, and it is common for a commentary to be commented upon in turn.
Interpretation is where positions are argued
Schools that accept the same authoritative text must locate their disagreements in how it is read. The commentary becomes the place where doctrine is actually developed.
A writer arguing this way gains the standing of the root text while advancing a position that the root text does not state explicitly.
The result is that the most original thinking in some traditions appears in works formally presented as explanation of someone else's words.
The genre has its own conventions
Commentaries typically quote a line, gloss unfamiliar terms, state objections, answer them and connect the line to the wider argument.
Each of these steps expands the text substantially, and the objection-and-answer structure in particular can extend a single word into pages.
Because the conventions are stable, readers trained in the genre can move between commentaries on entirely different works without relearning the format.
What the growth preserves
Commentarial layers record disagreements that would otherwise have vanished, including positions preserved only because someone wrote them down in order to refute them.
They also document how meanings shifted over time, since a gloss on a word is direct evidence that the word had stopped being obvious to the readers of that generation.
For historians the commentary is often the more informative document, telling us what a community found difficult at a particular time.