I can't think of a reason the interior wall thickness would be influenced by the exterior walls or the roof structure. You need to consult a professional in the region.

Our house is concrete. The outer walls are 16.5" thick. The interior walls are normal size using 2x4s and drywall. I never considered that they should be thicker because the exterior walls are thicker than normal.

What is the purpose of such thick concrete?

I have seen houses with high ceilings put in extra thick walls. They can look a little spindly when they are only a few inches thick and are 12'+ high.

However, it's only an issue where the end of a wall is visible, like the edge of a partition wall. If your wall wraps around a corner, or has door casings, the thinness of the walls isn't as apparent. And since thicker walls take up square footage, and cost more to build, you wouldn't want to do it where it wasn't really obvious.

Renovator8 - If your question was directed to me, the outer walls are not all concrete, they are concrete, rigid insulation, stud walls and drywall. The reason behind the construction method is because it was built to be Passive House certified.

Thank you all for your comments. :-)

Here's an example of a structure with 24 inch outer walls. Skip the intro, when the name appears click on it then Projects > Built > Desert City House. There are some interior shots but I don't see anything that could be considered a "standard" interior wall.

http://www.masastudio.com/2012/