Different Collision used if "isDynamicObstacle" checked for navmesh?

Development World Creation
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See this actor. It’s a bit sinked into the ground because like this it’s easier to demonstrate.

This is how it looks like with isDynamicObstacle disabled:

http://puu.sh/gEOHB/0a21953090.png

This is how it looks like with isDynamicObstacle enabled:

http://puu.sh/gEOGp/cccf4a8546.png

I don’t care about the navmesh generated on top of the mesh, I just wonder why the size of the area where no navmesh is is bigger if the mesh is no dynamic obstacle. The mesh itself has no collision at all (I removed everything). Although if the mesh is a dynamic obstacle a different collision type is used to calculate where navmesh is and where not.

How can I change what collision is used for the navmesh? I need the mesh to be a dynamic obstacle, but then too much around it is marked as “nope, no navmesh here”.