Thin is relative to intent
Someone searching a product's maximum upload size wants a number. A page that explains why file sizes matter, then gives the number at the end, is thin for that intent even though it is long and accurate.
Thinness is therefore measured against what the reader came for, not against a word count. The same page can be substantial for a reader who wants an overview and thin for one who wants to configure something.
This is why the fix is rarely more words. It is usually a decision about which reader the page serves, followed by removing the material written for the other one.
A page that tries to serve a beginner, a practitioner and a buyer at once serves none of them, and reads as thin to all three.