this is because the 'final mix' is not the last step in the production process.
all commercially produced cd are then treated to a process called mastering.
so if you compare a mix that has not been mastered to one that has on a Cd or I-tunes, you will hear a big difference in volume, as well as overall Eq and punch.
during this process the overall level is seemingly increased via the use of program eq compression and limiting.
the good news is Logic offers some really useful mastering plug ins to do some of this for you if you want. you can get pretty good results with it, but it will most likely never compare to sending the final mix's to a mastering engineer.
to try it your self in logic 8 go to the output buss 1&2 (the one to the left of the mix buss on the mixer page ) and at the top of the channel strip where the 'settings' box is, you'll find in that there are channel strip settings with a section called 'mastering'. try one of these pre sets and see if it helps you get more smack and volume as well as brightness to your mix.
mastering houses have really expensive hardware that sounds amazing that they run the mix through to do this. I'm talking about eq racks that are like $5 to $10 k for 2 channels of program EQ. this is also exclusively what they specialize in and do day in and ay out, so there very skilled at it.
a big part of this process though is handing the reigns over to someone else for a second opinion who is not biased for a different perspective on what can be improved on to make the whole cd better. kinda like a second opinion that is listened to in a different room. there listening rooms should always have an amazing sound system in a tuned room which will show any flaws a mixer may have missed in there, perhaps not as good, mix room.
I personally have had great results with mastering guru Brad black wood over at:
heres a one of Brads web pagers that explains it for you:
or more info on this subject you could look at www.gearslutz.com in the mastering forum: