We carry what is necessary. When I went from infantry to special ops, my pack got bigger and heavier.
When OIF started, my unit was responsible for taking the western desert. We had to pack enough food, water, ammunition, to get through to the next supply drop. We had the best gear available to the military and did not carry luxury items.
We were fortunate enough to have vehicles so humping these packs was not necessary but we always trained for it as a contingency plan.
Training marches were a 75 pound ruck ( and water was extra on top of this weight) we had a little supply guy who weighed about 130. Despite maxing his pt test these marches were an epic struggle for him. He would constantly fall down because the weight was too much for him.
Different units have different missions and physical demands.
Most of the guys in my unit looked like big country boys but we were suitably built for our mission.
My brother-in laws unit down the road LRRSD, they were younger, and generally smaller than we were and they were suitably built for their task.