Hmm, the way I understand it the thinner the air the faster you have to go to get the same mass of air over the wing to create the same lift. The harder your engine must rev because there is less airmass for it claw onto. This is simplistic know. So at cruise speed you will have the same resistance on the bar because you are flying faster to make up for the lower pressure.
But then having said that the airspeed indicator will register a lower speed than actual speed through the air because the air is less dense.
And apparently they cancel each other out. So if you normally cruise at 50mph in less dense air you will have to cruise at 55mph (true airspeed/ speed through the air) but your airspeed indicator will register 50mph still. The only indication you would have of this situation is the engine reving higher because there is less pressure on the pitch
And your GPS will read higher than your airspeed so you might think you have a tail wind component.
I think that about covers it, so I have no idea why the bar would feel lighter.
only saw afterwards with GPS.
but I hope you come right. 
