The maximum weight that a porter should carry is 20kg. This is the official Kilimanjaro park limit. Packs should be weighed at park entrance gates to ensure that porters are not carrying too much. Some of the weight will decrease over time as food supplies get used up. As climber bag weights usually do not decrease, responsible operators limit the weight to 15kg.
Unfortunately, too many unscrupulous companies force porters to carry excess weight. Furthermore, when guides and park wardens all struggle to make a better living, the temptation for bribery is high. In the most unfortunate cases, porters have even been found carrying packs weighing up to 35kg.
Also, when porters who are not adequately prepared for the harsh mountain conditions fall sick on the mountain, their guides may not arrange for a replacement porter, so other porters have to take on the extra weight.
Good tour operators will ensure porters are carrying no more than 20kg and distribute the weight fairly among their porters every morning.