So I've stripped away some of the parts of the coupe within the game and exported the JBeams. My questions is why are the exported JBeam files so large as opposed to the original files?
You should edit .jbeam files in NodeBEAM Editor. Im using 0.37 version. Exporting .jbeam from blender isn't good idea, because after that exporting game is crashing (my experience). http://www.beamng.com/threads/nodebeam-editor-v-0-37.6103/