DTH drill rods (also called as drill tubes or drill pipes) are the mechanism to transmit impact force and rotation torque to DTH hammers and bits, as well as offer pass for the air flow.
In principle, the lighter the rod is, the better it is for the drilling operation. So a thinner one is always preferable than a thicker one if other parameters were the same. While in the mean time, the yield strength and the tensile strength must also be taken into accounts, which means relatively rod wall thickness would be needed to obtain enough strength. Of course, once the thickness is rigidly requested as thin as possible to minimize the drill string weight, there is also another method to get optimum drill tubes that using better grade steels.