bigger window == larger area == more pixels

more pixels == higher bandwidth either across the buss and/or on the graphics board. it also means more processing as the scene is rendered, causing an increase in cpu usage

you could reduce the effect by radically reducing the maxfpsbk ... 2 or 4 would be good enough

try reducing the screen res... it would also give you less pixels to render/ generate/ push ... yielding better performance and lower cpu/ gpu usage