Size isn't all that matters in determining how much SPL will be put out. How far the diaphragm can move and with what force the magnets can move also matters. If you have a giant diaphragm and it only moves a small distance then it is not going to be nearly as loud as if a similar diaphragm is moving much further and displacing more air (stronger pressure wave).
Usually that's right, given that you plug both of them to the same source. The headphones size has nothing to to with it. Headphone impedance is usually increased by wire thickness and wire loops. While more loops creates a larger field and thinner wire usually works out to a lighter, more responsive diaphragm, resulting in a more accurate response.
Few facts:
That actually means that low impedance headphones usually used for portable players since they can provide less voltage. While high impedance headphones usually used for professional purposes with equipment with sufficient voltage source.
