The inductive loop is a big, low power electromagnet, with no core (or with an "air core" if you like). It operates as an inductor coil normally and an electrical signal with a certain carefully selected frequency is circulated through it. When a conductor is placed within the electromagnetic field of the loop the magnetic field properties change due to induced currents in the conductor. This changes the frequency response of the inductor, a signal that can be detected by the monitoring equipment.
What matters is primarily the surface area of conductive material (metal) near the road surface. A huge lifted truck actually generates a weaker signal than a tiny low clearance sports car. Some vehicles have a very small horizontal cross section near the ground (bicycles and motorcycles) so are not as easily detectable.
Also, it doesn't matter whether or not your car is running, if it contains a substantial amount of metal it will generally be detected.