Bonsai are full-fledged trees, and contrary of some folk beliefs, they are not genetically selected and modified to grow smaller, nor do they stay small because they are not watered or fed enough.
Bonsai are followed meticulously for their entire vital cycle, a perfect balance between the apex and the roots is maintained through pruning, fertilization, and other common techniques used for protection and care of plant diseases. Bonsai keep their reduced dimensions, or to be more precise they slow their growth to maintain the right ratio between foliage and roots; This is possible because they are planted in a pot and are not in direct competition with other plants.
Although these practices can appear as invasive to the plant, they are not any more so than pruning an orchard or taking care of a dog: it is the overall state of health of the tree that expresses its well-being. Moreover, techniques such as leaf trimming, root pruning, repotting and wiring are meant to sustain the form of the plant, to increase the exposition of the foliage to the sun, and to rearrange the root system in a better fitting space, thus renovating the terrain nutrients and increasing the long-term health of the tree.
If a bonsai is thoroughly cared for during its growth, its lifespan can be increased up to hundreds of years. The rule-of-thumb for bonsai specialists is “do not do to the plant anything that you need, but only what is best for its health: you are not the owner, but the caretaker”.