To export an FBX file
Geometry is selected on the basis of taking the most obvious objects in preference to the fine detail. If the exported file is too large, try enabling this feature and reducing the number of polygons. Clicking the Home tab, sliding out the Project panel, and clicking Scene Statistics in Autodesk Navisworks shows the number of triangles and lines in the current project. By setting a polygon limit you are choosing to export only some of these items.