So, I land my medium chopper on the helipad, set the throttles to idle, get out of the helicopter to have a look around, when I get back in, the engines are running at full throttle. I tested this several times.
I also noticed that If I run the starters on the medium chopper, then release them without engaging the throttle, the engines don't slow down until I press the throttle closed button. Don't know if that is a bug or intended, though.