We have a vBulletin board sized similar to yours. We've had as much as 91 users online at any one time, and a fairly rapid growth rate in terms of message load (close to 1,000 in the last week or so, for example). We've never seen our resource usage climb to anything that isn't acceptable here and performance is pretty good almost 24/7.
So, yes, I agree that the user in question must be doing something strange, such as running unusual mods or scripts, and/or providing videos or other huge downloads.
We consume more bandwidth than both of you combined, but that's because of the Podcast version of our radio shows, so it's not very resource intensive.
Co-Host, The Paracast
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