They were mainly full of water and MRE boxes and personal gear; duffle bags, ruck sacks, sleeping bags and the likes. The crew carries quite a bit of gear. The crew is normally 9 guys; each with one duffle bag, ruck sack, sleeping bag, sleeping mat, etc… The planning factor was also to be able to go at least two weeks w/out a water/rations resupply. That equals 5 gallons of water per man per day, plus 3 MREs per man per day. Once added up, that is a lot of water and MRE boxes.
As a side note, only 2 BDE, 3 ID in OIF 1 (invasion, March '03) had the side racks. These vehicles were drawn from Pre-Positioned (PrePo) stocks in Kuwait and had the locally made side storage racks.
I made one a while ago; you can follow along here.
You can check out the M2/M3 Bradley Reference Archive thread for general Bradley info as well.