I'm not entirely sure, but as an extreme example, imagine you placed a small ball in front of a light source,behind the ball is a white screen. The ball will cast a shadow on the screen. Now imagine putting a small pebble directly behind the ball. Provided the ball is much larger than the pebble, would you still be able to see the shadow of the pebble? No. The large ball's shadow blocks the any shadow of the pebble from forming. In this scenario, you would need something smaller than the pebble. For Rutherfords experiment, I believe what he did was shoot alpha particles at a gold foil. Relative to the gold atoms, the alpha particles were much smaller (the size of a helium nucleus) and so they were able to pass through the electron cloud into the fluorescent screen. Occasionally, alpha particles would hit the nucleus and fly back, but the important thing to realize is that most of those alpha particles would pass through, and those that did displaied a "shadow" of the nucleus on the screen, since the alpha particles passed through the electron cloud (but not through the nucleus). This gave Rutherford some idea of what atomic structure could look like.