You may want to try Bandwidth Daemon
You can define filters to specify what traffic to monitor, it looks like you can do it based on IP so you could set monitoring rules for your most used websites to see how much traffic they're dumping to you.
As a bonus, the trial period is 30 days - just enough time to find out where your bandwidth is going
Also, you may want to check out 3proxy
, files are hosted on Sourceforge if being a Russian site discourages you.
It can do logging of data received from destination IPs outputting to a format defined by you, (check it's documentation on Sourceforge), including Squid compatible, so you can use log analysers. Or output to a CSV and load it into a spreadsheet to work out.
It should be a simple matter of installing it and then setting your browser to use the local proxy, (at least that's what I remember when I used a local proxy).
Of the two methods above, the local proxy would probably be the most useful at determining where your data has gone.
PS. You're not a YouTube junkie are you ?