Replying to Headphone power drives?

  Ok, so i'm looking for some new headphones and i check out the skull candy site and i notice that in the description they would say: "50mm powerdrives", "40mm power drives" and so on.

  what i'm wondering is what that means. is that the quality of the sound or just the raw power of the sound? like, would a 50mm be louder than a 40mm out of the same mp3 player?

 and on another note, how can i get more sound out of my mp3 player. I have one of those sony walkman mp3 players and when i use earbuds its alot louder than these earmuff type ones i've got. but when i plug them both into my stereo the earbuds sound like thier gonna explode, while the other ones pull out a loud good quality sound. is it just a question of the mp3 players built in amplifier or does it also have something to do with the headphones