Being small is not in the question. Being there is. Otherwise we wouldn't be here discussing all hyper dependencies in detail as some of these are a lot smaller than zlib.
I am late to reply. What I mean to say is zlib is a small self contained dependency, suitable for static linking, and no major dependency of its own, just math. A lot of more modern libraries and languages tend to have their small dependencies pull in a massive hydra of dependencies.
TLS implementions are often giant hairballs but one that many things depend on. You can think of it as a somewhat "system level" dependency.