[HAL9000]

We took this up in Physics years ago in high school. Rubber is not a solid material. Under a powerful microscope, it looks like a bunch of strands of material randomly woven together. There are molecules in "air" that are small enough to escape out between the gaps between the strands. Nitrogen molecules are the largest in the air composition and can still escape, but because of their physical size, it is much more difficult, therefore the tires don't leak. Thicker tubes mean more strands for molecules to work their way though, less chance for leakage or slower leakage.