The graph isn't uniformly distributed by radius, for one thing, as it has 0.5R-3.1 Re divided into four different bars, with 3.1-22 Re divided into the other fo

The graph isn't uniformly distributed by radius, for one thing, as it has 0.5R-3.1 Re divided into four different bars, with 3.1-22 Re divided into the other four, so it wouldn't look like a bell curve even if they were normally distributed as it's designed to highlight planet sizes that are closer to earth sized. Also, as someone mentioned, Kepler is only likely to find planets fairly close to their star. And if you have several large planets that are close to their star, they are more likely to interact and be knocked out into a higher orbit until only one is left to dominate the orbits close in. Smaller planets are less likely to disturb the orbits of their neighbors which means a system is more likely to have several smaller planets closer into the star, which again, makes them more likely to be detected by Kepler. [edited for clarity]