Coaxial cable diameters range from 1/8 inch for short connections of a few feet, up to several inches to connect a broadcast TV station to its antenna 152metres / 500 feet up a tower. The terms "thick" and "thin" could mean different things to different users. To a computer technician, 1/8 inch could be thin and 1/4 inch, thick. But to a broadcast engineer, 1 5/8 inch cable would be thin and 4-inch would be thick.
The diameter of a coaxial cable measures across the outside of the shield, not to the outside of the protective jacket. The distance between the centre conductor and the shield determine many of the characteristics of the cable, including signal loss, the usable frequency range and the maximum voltage or power the cable can carry without arcing. Although many other characteristics apply, the distance between the two conductors mainly determines the thickness.
Electric capacitance develops with two metal parts placed near each other without touching. Therefore coaxial capacitance develops between the centre conductor and shield of any coaxial cable. The three factors that determine the amount of capacitance are the distance between the two metal objects, their size and the material placed between them, called the dielectric. So the distance between the two conductors contributes to the cable's capacitance and therefore its thickness.
The amount of signal lost while travelling through a coaxial cable depends on the frequency of the signal, the DC resistance of the conducting material and the capacitance of the cable. Typically, thicker diameter cable has lower loss and will carry the signal over longer distances. But the loss in the same cable increases as the frequency of the signal increases.
For thinner cable used mainly indoors, the insulating material between the centre conductor and the shield may be plastic, rubber or Teflon. The effect on the capacitance varies with each material, varying the distance accordingly. Thickness of outdoor cable will also vary to accommodate materials to separate the conductors. Larger broadcast cables use dry air for a dielectric, solid copper tubing for both the centre conductor and shield, and foam or occasional Teflon separators to maintain a constant distance between them.