A quick guess at the problem: it's probably overflowing the maximum amount of point/polys that an rFactor GMT can contain.
Each SObject creates one GMT.
As a guide you should limit the size of a SObject. This is improtant for two reasons:
1. It avoids the problem you are having with rFactor export.
2. 4 x 250m walls will render better than 1 x 1000m wall because the Level Of Detail system will be more effective.
I'll add a test upon export to at least skip long SObjects rather than export them.
The reason I don't prevent them in the first place is because RBR isn't affected by this problem.