The problem is less the size of a single dependency but the transitivity of adding dependencies. It used to be, library developers sought to not depend on other libraries if they could avoid it, because it meant their users had to make their build systems more complicated. It was unusual for a complete project to have a dependency graph more than two levels deep. Package managers let you easily build these gigantic dependency graphs with ease. Great for productivity, not so much for security.

The size itself isn’t a problem, it’s just a rough indicator of the benefit you get. If it’s only replacing a hundred lines of code, is it really worth bringing in a dependency, and as you point out potentially many transitive dependencies, instead of writing your own? People understood this with left-pad but largely seemed unwilling to extrapolate it to somewhat larger libraries.