Usually, it is a question of machine weight and its ability to hoist:
Weight limits in moving machines like elevators are typically calculated based on a number of different factors.
First is the weight of the machine itself; how it is hoisted, which is to say whether it’s using hydraulic lifts, cables, or other machinations, also makes a difference, as does what it’s made of.
The stronger the elevator the more it can usually hold, but only to an extent.
A very heavy chamber made with reinforced steel or lined with granite can usually hold less since it weighs so much empty. In most cases, the upper weight limits represent the most the chamber can weight at all, passengers and fixed material together, in order to operate in a safe and consistent way.
About person capacity:
Generally speaking, a standard elevator in a low-rise building can hold anywhere from 2,100 to 2,500 lbs (907 to 1,134 kg). The larger the building, the more capacity it will normally have.
In a mid-rise building, for instance, elevators can usually hold more like 3,000 or 4,000 lbs (1,361 – 1,814 kg), and in high-rise buildings, the limit is usually closer to 4,000 lbs. Most of this has to do with the strength and integrity of the machinery needed to life the chamber up high for sustained periods of time.
There is naturally more reinforcement in a shaft that ascends 30 floors than in one that hits just 3, for instance.
Statistically, a standard elevator is relatively safe. These machines are supported by steel cables and, even in small structures, these cables alone can usually support a fully loaded elevator car.
A picture of another home-elevator concept: