Kepler co-investigator Dimitar Sasselov revealed that the Kepler Space Telescope has discovered 140 potential Earth-like planets:
"You can see here the small planets dominate the picture," he casually said while referring to a graph depicting the different exoplanet sizes and their number as of July 2010.
Although he refers to these exoplanets as "candidate" Earth-like worlds, Sasselov goes on to talk about the statistical prevalence of small planets throughout the Milky Way.
Before Kepler, only the larger exoplanets could be seen. This is fairly obvious; large gas giants are easier to detect over the great interstellar distances. The highly sensitive Kepler has now leveled the playing field, indicating that there are many more exoplanets twice the size of Earth and smaller.