You cannot make an object stronger or stiffer by removing material. Using hollow tubes and flutes allows gains in strength and stiffness relative to mass, not diameter. A solid cylinder of a given outside diameter will be stronger than any tube of that same diameter assuming like material composition. The thicker the walls of a tube, the stronger the tube- again, given the same outside diameter. When the walls get so thick that they touch in the center and you now have a solid cylinder, it has become as strong as possible at that diameter.
