First make sure both your main output and your master fader are at unity (0 dB)
Are you using any metering, and if so, what does it say? If not, start doing that. You should at first aim for peaks around -3 dBFS, and for an RMS value of between -20 and -12 dBFS (depends on the genre).
If Normalizing is off (it should be) then your bounce will be as loud as you hear it. However, if it sounds quiter in iTunes, then iTunes is NOT set to unity gain (which is simply setting the slider to max)
Also, getting a mix "commercially" loud is not simple, brainsurgeons and rocketscientists have failed at it.
But here are some valuable resources:
First, the legendary "Lagerfeldt PDF" found here:
http://www.logicprohelp.com/forum/viewtopic.php?t=57055
then, these pages:
Holger Lagerfeldt
https://www.gearslutz.com/board/music-computers/468170-loudness-when-producing-m ixing-tips.html
http://www.logicprohelp.com/forum/viewtopic.php?p=282853#p282853
Mo Volans
http://music.tutsplus.com/tutorials/how-to-master-a-track-in-15-minutes-or-less- -audio-24
Tarekith:
http://innerportalstudio.com/guides/