Domination

Due to certain physical advantages that relative giantesses possess, they are more powerful than relatively smaller people, therefore, it is not surprising that they are inevitably physically dominant as a result of their brute force.

Some clarifications

 * That they are physically dominant does not imply that they have a dominant personality.
 * That they are physically dominant does not imply that they are psychologically dominant (for example, that a giantess is a servant of someone relatively smaller).