Brian,
The theoritical maximium that a soldier should be able to carry while maintaining a good pace and adequate energy for fire and manuever after a day of marching is 1/3 of their body weight.
Now that's a fairly laughable number. Our unoffical motto was "The rucksack is never full". Most of the time, as light infantry, we were toting 110-120 pounds rucks, while weighing in the mid 180's on average. The Romans also aimed for that 1/3 number, and never got it after Gaius Marius redid their packout.
That said, in the military you are never carrying just your stuff and you are never carrying the lightweight high speed gear either. They can't afford it, and won't waste it on grunts when they can. A cheap canvas shelter half, so fungus infested as to be dangerous and as heavy as hell will last to eternity. A nylon and spectra fiber bivy sack, light, warm and clean, won't last a campaign under the unthinking abuse a careless soldier ( I was one, once) will give it.
There are dozens of other examples, ranging from the weight of MRE's compared to the weight of jerky and a bottle of multi-vitamins, to the weight and caloric expenditure of a pair of cheap combat boots for warm weather compared to the weight of comabt sandals. Soldiers' equipment has to be designed and purchased for the lowest common denominator.
By spending your own money wisely, caring for your equipment, and training properly, you can not be that lowest common denominator and you can thus shed dozens of pounds off your civillain or spec ops pack.
My hiking/climbing/white water/ serious bushwaking gear load ( everything on me) , including two weeks of food and full hydration bladder, hovers around 35 pounds for three seasons in Colorado. Add another 8 pounds for two weeks of winter camping, not including snow shoes. With those loadouts, I can actually stay out a whole lot longer, although my movement rates drop as I am forced to hunt and gather as I move and I quit climbing due to excessive wear on the equipment and me.
Note that is is not by any means a "combat load" although it does include a sidearm, and I have spent a lot of money on my equipment and time learning how to use it.
Take Care,
Jeff