CB itself does not eat lots of ressources, as well as Joomla. This site is on a standard shared server, while Trail's is on a dedicated one.
It's not the number of registered users which give you the server sizing, but the number of concurrently browsing users, and even more importantly what they are doing...
Some easy-looking components like calendars are heavily loading the database, with over 60 requests for just displaying the calendar, while others are savy.
We have designed CB to be database-accesses savy, with one acceptable compromise: computing and displaying the link (degrees) between two users on a user profile is a heavy relational SQL request. That's why we limit it to 4 degrees links.
You need to take a careful look at the image gallery component, calendar, etc, as well as on the tabs that you use, more than on CB itself.