In addition to the different loadouts for different services, units, and missions, the contents of an individual soldier's pack are going to vary a lot from individual to individual. Just to get an idea, take a look at this list of extra gear recommended for a soldier deploying to Afghanistan. Now, a lot of that stuff won't get carried in the field, but some of it will. Soldiers carry a lot of stuff in addition to what they are issued (and they substitute better personal gear for some of the issue stuff).