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AT&T (s T) said that its average DSL subscriber only uses 18 GB of data per month when it announced its 125 GB cap earlier this year, and the company’s spokesperson Mark Siegel even called the caps “generous.” But new data published by network management specialist Sandvine this week might make one question the company’s rosy take on its bandwidth caps.

Sandvine said in a special report titled Netflix Rising (PDF) that the average Netflix user consumes about 40 GB of bandwidth per month. However, consumption seems to be much higher when Netflix is consumed with a connected device capable of receiving HD streams. The company singled out owners of Microsoft’s (s MSFT) Xbox 360 in particular: Users that stream Netflix through their Xbox 360 consume about 80 GB of data per month on average.

Unfortunately, we are not just talking about a few heavy gamers with Netflix co-dependencies: Around 25 percent of all Netflix traffic is consumed through the game console, according to Sandvine, with 33 percent of all Xbox 360 game consoles being used to stream Netflix content. One has to wonder how many of those Netflix-loving Xbox owners are poised to hit AT&T’s bandwidth cap with their next movie marathons.

Sandvine also compared Netflix traffic to other sources of bandwidth usage, pointing out that the video subscription service now uses close to 30 percent of North America’s peak bandwidth, surpassing BitTorrent as the largest source of data traffic in North America.