Why are most discovered exoplanets heavier than Earth?
There are a number of methods of detecting exoplanets, but all of them favour detection of larger planets over smaller ones, albeit for slightly different definitions of large:

Radial velocity measurement — this detects the small movement of the star towards and away from us as the planet and the star orbit their mutual barycenter. This movement is fastest when the planet is massive (so the barycenter is further from the center of the star) and close to the star (so the orbital velocity is highest). I also needs the planet's orbit not to be &quot;face-on&quot; to the Earth. This method produces measurements for the $mass\times \sin(i)$ since a more massive planet in a less inclined orbit produces the same motion as a less massive planet in a more inclined orbit

Transverse displacement — this detects the small movement of the star from side to side (against the background of distant stars) as the planet and the star orbit their mutual barycenter. The displacement is largest when the planet is massive and far from the star (although distant planet require observation over a long period of time).  It works best on stars close to us.

Transit — this detects the tiny reduction in the brightness of the star when the planet moves between us. It is more likely to detect large planets, and more likely to notice if the orbital period of the planet is fairly small.