A lot of it really does matter on steel chemical composition and heat treatment.
The measure of strength when you’re talking materials is something called yield strength which is given in terms of pounds/square-inch. This is the amount of stress that the material can take before it begins to yield (i.e. plastically or permanently deform).
A lot of solid round bar, such as re-bar, really isn’t meant to be a structural steel, and as such may be pretty weak.
Whereas steel tubing is usually used in a structural application (as opposed to copper or brass tubing which is pipe), so it tends to have a higher yield strength.
So it would be easy to have a case where a hollow steel bar is stronger than a solid steel bar, it’s just that the two materials were not identical, which was once of your constraints.
I know that donkeyoatey mentioned that they were both mild steel, but that term doesn’t really have a lot of meaning. You need to know things like carbon content and manufacturing process to really compare the two.