Why are some headphones louder than others?

For several months, I owned the Sony "sweet little earbuds", and when the sony earbuds died, I bought a pair of the Skullcandy ink'd earbuds. What I noticed is that with the skullcandy earbuds, I didn't have to turn up the volume much on my ipod, while on the Sony earbuds, I had to turn up the volume. What factors affect the "loudness" of the earbuds: the impedance, or its max input?