Why is this folder 4.2mb? It only has a handful of relatively small text files in it! I have a WordPress plugin which was originally 26kb zipped (79kb uncompressed).

I installed it, then made some live changes, then downloaded it and found that the plugin folder is showing 4.2MB in Finder!

Expanding all the folders in one Finder window, I can confirm that there is maybe 50KB in there unzipped.

What is going wrong? Even when I zip it it's 3.8MB. Where is this extra data coming from? Right now I'm blaming the Mac, but maybe this is wrong.
Did you check hidden files? What about https://en.wikipedia.org/wiki/.DS_Store? :)

Open terminal, go to that directory and type ls -alh. Then compare what you see in terminal and what Finder is showing.