Most of the planets that have been discovered using the transit method have had large diameters. Why?

Most planets discovered using the transit method have large diameters because planets having large diameters results in a more pronounce (noticeable) dimming effect during their transit between a star and an observer and are therefore readily discovered using the transit method than planets that have a smaller diameter

Explanation:

The transit method involves the detection of a transit (dimming) during the motion of an object between a star and an observer such as the motion of Mercury or Venus between an observer on Earth and the Sun

A large planet gives a higher dimming effect than a small planet and they are therefore more easily detectable using the transit method

The transit method is the method used to discover most of the known exoplanets which are planets that revolve around a star which is not the Sun

As the planet transits in front of a star in an observer's line of sight, it results in the dimming of the stars light by an amount equivalent to the ratio of the planet's to the star's circular areas

Example of planets discovered using the transit method includes the planet HD209458b.

what's the