Mostly it is due to weight, size, and the person carry the ammunition and the weapon.
If you give a Spanish Model 1916 Mauser to someone that weighs 110 lbs and is only 5'2. The recoil will be harsh for someone of that size, also the weight of the rifle, ammunition, and other items needed for that soldier will wiegh him/her down.
Now, if you give someone a M4 Carbine or G36. It will weigh a little less than the Spanish Model 1916 and the recoil will not be a problem. Also, more ammunition can be carried under the same amount of weight.
It comes down to numbers basicly. Weight, amount of ammunition, and the size and stature of the person using the rifle.