Answer to Question 1
Earthgrazers have perihelions inside the Earth's orbit, and thus could potentially hit the Earth, crossing our own orbit, as well as those of Mars, Venus, and even Mercury.
Answer to Question 2
The differences in the sizes of the jovian planets are due to their capturing different amounts of gas from the solar nebula, since their cores are all about the same size. Icy planetesimals took longer to accrete in the outer solar system, because they were more spread out there. Thus, more distant jovian planets didn't have as much time as Jupiter to capture gas from the solar nebula before the nebula was cleared by the solar wind.