Commented code != bloat
PHP scripts are pre-compiled, so comments in the code only slow down their execution the first time they are run. If this really bothers you, you could strip all the comments from the code before going live with your site. As it is, I have never gained any meaningful increase in generation time by doing this.
Producing open-source software is a good excuse to comment your code, as people might want to re-use the code you write. I don’t want to waste time trawling the web for an explanation for a particular peice of code, when the function (as coded) has no english-language indication of what it does. I’m not “having at it” or “spouting off”; I’m pointing out a problem with WordPress.
What if I want to do more than adjust the templates of my site? For a start, I added the functionality of attaching uploaded files to particular posts in my WordPress site; it took considerably longer to program because the code is not commented. I don’t doubt the code is documented somewhere, and you can probably find out how to make common changes quickly, but if you want to go deeper it becomes quite frustrating … or maybe I’m just lazy?