I don't know if the smaller body and possibly less heat dissipation result for less efficiency. For one thing, there might be typos in the runtimes tables - if you compare the new XT11 to the old one, you'll see the older one has much better runtimes on low.
In any case, assuming the runtimes provided by Klarus are accurate, there are other possible explanations on the efficiency - i.e.:
Klarus XT11 is an XM-L2, I think I T6 but I might be wrong. The one on the Klarus XT2C is an XM-L U2 (first generation) so the XT11 is currently one LED generation and at least 1 bin up, maybe more, on the latest XT2C with 580 lumens.
Klarus might have updated the XT2C to XML U2 and just driven it harder on high without changing much. For the XT11 they might have gone furhter and replaced driver and or twicked the electronics not only to drive it harder on high but also to keep efficiency on par or even improve it.
I think the only ones who could confirm it with certainty are Klarus: you could try to write to them but they might not reply, reply to a different question, tell you they only sell to distributors, or even to check your batteries...
In other words, they are not the best at communicating with customers. Luckily their XT lights seem to be well designed and work fine!
In any case, I think you made a very nice buy there. I have the XT2C XM-L T6 too and is a nice light and I find saving 20 bucks to be smart move when 400+ lumen will do more than fine for most uses and give you a longer runtime! For the uses where they might not be enough, 500+ probably wouldn't either anyway! Klarus might come out with an XT2C with XM-L2 at some point so you can always upgrade then if you really need too.