Do you increase the number of wheels when you increase the vehicle size? Having more wheels means that more raycasts are issues by the vehicle, which might be the source of the performance drop. Having said that, I wouldn’t expect a noticeable performance difference until there were hundreds or thousands of wheels.
It is possible that moving a very large chassis around the world is having an impact on the time spent in broadphase and narrowphase (collision detection). One way to test this is to create one of your large vehicles but replace the shapes you add for the vehicle chassis with very small shapes. Alternatively, you could just disable collision on the chassis shapes [(PxShape::setFlag(PxShapeFlag::eSIMULATION_SHAPE, false)]. Again, I wouldn’t expect a single large shape to be a problem but if you have hundreds of large vehicles the cost might accumulate. How many large vehicles are there in your game?
Could you please provide more details about the type of geometry that you have and how many instances you need to see a drop in performance? A really quick way of doing this is to record your simulation in PVD and then post it here so we can take a look at it. PVD records timings of the various update components in the sdk so will provide valuable information.