As the threat facing British soldiers has changed so has the composition of body armour, which now consists of front, rear and side plates designed to protect soldiers from small arms fire and IED blasts but weighs around 40lbs.
In addition to body armour, a typical soldier on patrol in Afghanistan will carry: a weapon (10 to 20lbs); radio, batteries electronic equipment (40lbs); water (10lbs); ammunition (20lbs); Javelin missile (25lbs). Soldiers will also be required to wear eye, groin, ear and knee protection as well as gloves and a helmet
A USMC radio operator on a ground patrol with a plan to be out for less than a day and cover 20km will have radio batteries, a radio, flak jacket, 4 extra magazine, m4, smoke grenades, helmet, sunglasses, weapon cleaning kit, extra socks, 2 days of mre, probably 2l of extra water, poncho liner, rain jacket, gloves, extra socks. Others will have heavier weapons but less radio gear and two people will have ied jammers. Night vision goggles and a peq 16 plus extra batteries and grenades will depend on what they are doing. 60lbs or more is normal for a ground patrol.
Soldiers (And other military members) are expected to be self-contained for the entire time they're in the field, with enough food, water, clothes and other gear to cover the time they're out and then some. When you're in hostile territory, you don't want to run out of anything you'll need, so many soldiers overpack.
That was the lesson of the movie Black Hawk Down. Many of the Rangers didn't take water, body armour, night vision etc because they thought it would be a quick in-and-out job.
I'm guessing after some quick math that its not the water like I originally suspected but perhaps the armor and ammo but maybe not?