I have a small variety of Dunlop Tortex pick sizes. Normally, the 0.73mm (green) is my favorite. When I recently picked up my first acoustic in ages, I chanced to discover that the lighter 0.56mm (I think, red) sounded sooooo much more brilliant. The lighter pick seems to make it sound brighter (for strummed chords). Why would a lighter pick create a "fuller", "brighter" sound?
EDIT: I should add that my electric guitars use 0.010-0.048 EB Slinkys, while this acoustic is strung with Martin "light" gauge strings.