How to track down which files account for my outbound traffic?

There wouldn't be a way to track which files are being served specifically, but you could use a network monitoring tool (such as one of those mentioned in the below post) to monitor your outbound traffic:

http://www.linuxandubuntu.com/home/best-network-monitoring-tools-for-linux

Depending on which tool you use, it should give you some helpful information regarding TCP flags, ICMP details, TCP / UDP traffic faults, TCP connection packet, etc.

How to track files that are served to account for 19.6GB transfer?

This question is interesting to me also and I thought there is a way to gather page statistics or something?

@Linode

Confusing name as it make me think I'm talking to Linode the company.

@jcardillo

Forgive me if I'm wrong but I was so interested in the question that I wanted to dig a little deeper. Now, I have no real experience with addressing what the op might be talking about (and I'm not really clear on what his question is) but I found something I was curious about myself that I thought addresses the op's question.

For nginx..

nginx-module-vts

Screenshot

Or

For apache..

Apache Module mod_status --> "A total number of accesses and byte count served"

"The server-status page may be used as a starting place for troubleshooting a situation where your server is consuming all available resources (CPU or memory), and you wish to identify which requests or clients are causing the problem."

and..

How to Monitor Apache Web Server Load and Page Statistics

@Linode

Does this address your question? Perhaps I'm totally off base and misunderstood what was being asked for?

Thanks
Jake

PS: Perhaps the network transfer statistic shown in linode manager also includes usage from ssh by the user? Not sure; but, if it does that could alter the result and need to be differenced (subtracted) to get a pure result.