The more experienced the soldier, the less ammo he carries. From a personal perspective, when I started out in Bosnian campaign I carried as much as I could, which was about 8 or 9 mags with 30 bullets each. After a while, however, I realized that I rarely shot more than ten bullets during a firefight and I never ever spent more than one complete magazine. So I asked myself why was I carrying so much ammo around.
Later on, during major offensive operations, I used to take only four mags with me: one in the rifle, one in a big side pocket of my trousers and two in my combat vest. Additionally, I always carried some loose ammo in my trouser pockets.
Instead of more mags, I put two cans of baked beans or something similar to my mag pouches. After a day of marching through the woods and hills of Central Bosnia, I sat down and had something decent to eat, while the rest of my unit was munching on some army hard cookies and MREs.
Much later on, in Kosovo, I went down to three mags. In cases when we operated far away from our base I took another two magazines with me. This was absolutely sufficient for such kind of operations we used to run.
I already had to carry hand grenades, rifle grenades, and sometimes an RPG. Better to have an extra rifle grenade (that you might actually use) than to return with four full mags to the base. Although a single magazine doesn’t seem too heavy and won’t take much space, three or four mags are a completely different story.
The less you carry, the better: you are more agile, less exhausted and therefore more awake and attentive. Short, you are a better soldier. That means that the well-trained soldier is far more effective than the one who is less experienced and better equipped.