Stiffness is (almost exclusively) the result of tubing diameter.
By way of a somewhat ridiculous example, compare the following two pipes, both 2m long:
- 25mm diameter tube with 6mm thick walls.
- 100mm diameter tube with 1.15mm thick walls.
Both pipes have the same cross-sectional area and therefore the same weight per meter. Which one will be stiffer?
An online calculator estimates that with at 10kg weight suspended in the middle the first pipe would deflect nearly 70cm and the second pipe less than 3mm.
That example is a bit extreme (very thin walled tubing is prone to buckling when loaded), but it does demonstrate that diameter trumps wall thickness when it comes to structural stiffness.