I have a similar problem to that of the user who posted High memory usage of WordPress hosted on DreamHost VPS. The reason this is a 'problem' is because when it maxes out the memory quota I set for my VPS it will begin serving 503 error pages for 'uncached' pages (I guess?). Setting the quota higher does help, but I thought nginx would save me some money as well as provide better performance and less memory usage than Apache (especially) and lighttpd. One of my sites is simply an HTML-only website, and the other is a Wordpress website almost nobody looks at / extremely low traffic. I don't get it. I have the quota set to something like 512MB or 640MB RAM just to have it running decently.
Does anyone have any ideas as to what I could do to make nginx run more efficiently, or why I am having this problem?