Garry,
I'm not 100% sure, but I think the Light/Heavy setting changes the voltage sent to the trailer brakes at a given press of the truck brakes. Heavy would send higher voltage down the wire causing tighter grab of the trailer brakes.
One way to get a feel for how much your trailer brakes are doing is to get up to 40 mph, coast, and press the manual control on the truck's brake controller. If you press and hold the manual control, it's applying maximum voltage. Based on my experience, on level ground, if you hold it at max, the trailer brakes should slow you from 40 to 20 mph in 5-10 seconds. You can also try this from a stationary position. Ideally, if you hold the manual control, the trailer brakes should lock the trailer wheels. The truck can probably drag the trailer anyway, but on a new trailer, where everything should be working well, the wheels shouldn't turn.