I am new to jMonkey. Recently I was trying to make a vehicle run on a track. I found out that the fps is very low at that time. However, after I scale the track 2 times larger the fps increased significantly.
Yeah it should pretty much be that. More triangles fall into the range of checking for the car.
Also I would suggest replacing that hull your car has with something like a few boxes in a compound collision shape. The fps gain can be unreal as hulls with more than 6-8 vertices tend to perform really poorly (which is probably why you had those 11fps in the first place).