I have a macbook pro 15in. Every time I plug in my charger cable and thunderbolt cable from thunderbolt display and close the lid I get full screen resolution. If I take the charger out and leave the thunderbolt display cable in, the screen size shrinks. What is happening when I unplug the charger from laptop? Can I combat this problem with a setting? I really don't want to have to always plug in the charger every time I use my display for sake of battery life.