Servers (web, applications, etc.) such as Nginx, node.js and Gawn are designed to use the as few resources as possible. As they work asynchronously, they do not need to create a process or thread for each new request. Therefore, they do not waste their resources.
While most web servers increase their RAM consumption as they receive requests, asynchronous serves remain stable.
Nginx is known to be more efficient than Apache and can serve 2.1 times more requests per second.
Further reading:
http://nbonvin.wordpress.com/2011/03/14/apache-vs-nginx-vs-varnish-vs-gwan
http://nbonvin.wordpress.com/2011/03/24/serving-small-static-files-which-server-to-use