It is true that Kirby is often referred to as a CMS for small websites. But that is a misunderstanding, I think. Maybe more than words, use cases speak another language.
Two of the big sites I know of:
http://nimbus-lighting.com (@bastianallgeier)
https://www.joyent.com
Some others, that are more than just small portfolio sites
http://www.theater-erlangen.de/de
And I think, there are many more examples out there, sometimes hard to find, because they are not advertised due to non-disclosure agreements.
Also, Kirby allows you to integrate other services, maybe this use case is an interesting example of using Kirby together with other technologies: http://made.ginetta.net/siroop-jobs.html.
Check out this thread as well: Is it possible to "grow out of" a Kirby CMS?
I hope, this helps as a starting point.