Polylist maximum volume

I have been trying to debug the collision mesh on a vehicle I recently added to my game. I have narrowed down the problem to the scale of the vehicle (it's larger than any previous vehicles in the game): the polygons from its collision mesh are only being recognised when they're within a certain distance of the vehicle's centre. Even the wheels, added as boxes by the game engine rather than as part of the DTS file, are affected by this. If I scale the vehicle down before exporting from Max, it's fine; if I scale it up, more and more of the polygons are missed.

The shape looks fine in ShowTool, so I assume it's not a loading problem but a problem when querying the polylist.

Any ideas? I can't find any mention of distance being a factor in adding polys to a clipped list.