They're a CDN and send out a lot of bits, so every last bit of inbound traffic helps. Or am I totally off base on this?
I vaguely recall a site that that shows approximate throughput for given properties but I can't remember the terms to google for to (re)find it. But 1.1.1.1 might not be listed - or the numbers might only represent valid traffic and be wildly inaccurate.
Traffic breakdowns would thus be very interesting to see (for whatever is shareable, at least - heh, you'd probably have enough fireside stories to drown multiple HNs with :) ). In the case of 1.1.1.1 I'm wondering about what percentage of traffic is coming from valid DNS clients (and I also wonder where the requests are coming from - although that's just trivia to me), versus eg nonsense from horribly misconfigured lightbulbs.
I would have thought that they would go with the simplest measure possible, bytes in vs bytes out.
Most network providers have peering agreements to handle reciprocal traffic flows. In other words, if you're Comcast, you send a shit-ton of traffic to, say, Verizon. But Verizon also sends a shit-ton of traffic to you, as well. Generally, companies will have peering agreements that express the price for which they will route traffic for other network providers, and generally if there's a lot of reciprocal traffic, the peering will be "settlement free" - in other words, neither party charges the other to route traffic. So, in the example above, you as Comcast would agree to route Verizon's traffic across your network for free as long as Verizon did the same.
Cloudflare is a CDN, which means they push a LOT of traffic out across a lot of networks, and it's likely they're not getting as much inbound traffic as they're pushing out. That makes it harder for them to negotiate settlement-free peering, since they're not providing as much reciprocal value to their partners. By owning 1.1.1.1, they can now claim any trafic sent to that IP as "inbound" to Cloudflare's networks for the sake of peering agreements. Since 1.1.1.1 gets a bunch of traffic from either misconfigured equipment or people doing silly tests, routing that traffic helps improve Cloudflare's ability to negotiate better peering agreements.
Which, honestly, is pretty clever, since most of that traffic is garbage.
Nobody expects a residential ISP or a CDN to have balanced flows at part of a settlement-free peering agreement.