Assuming the mean is close to the median, they are reporting ~10B requests daily with a median response size of around 1KB. 10TB daily is a little under 1Gpbs. Traffic is spiky, but this isn't particularly complex once you consider they have multiple data centers/servers. Of course, I may have misread something as daily that was hourly or something like that...
So it looks like the root namespace providers output a totally reasonable amount of traffic. Divided between the hundreds of points of presence globally, this is tens of megabits per physical host.
The servers themselves are ordinary 1 RU physical rack mount servers with 1 Gbps or 10 Gbps Ethernet. Nothing special. I'm guessing that most of the load isn't from the root, e.g.: "j.root-servers.net", but from hosting the authoritative DNS servers for .com and .net (b.gtld-servers.net) on the same box. That would surely have more traffic and much more data.
