It was commented that there are currently 22 SpaceX upper stages in orbit that will eventually reenter in an uncontrolled fashion. PcMan pointed that out when I asked Why the Long March 5B wasn't deorbited intentionally, and it's a good point. If you add up all those stages, that's more mass than the 5B, I'd guess. I didn't check for the other GTO launch stages also in decaying orbits.
I'm leaving out discussion of how the difficulty of deorbiting many upper stages operating at their limits in launches to geostationary transfer orbit, compares to deorbiting 1 core stage that was pushed to the limit to put one thing in a low orbit. I'll likely ask about the difficulty of deorbiting later.
I'm just looking to understand how the amount of debris compares. The Falcon 9 upper stages will come in with more speed and are far smaller objects. How does the amount of debris that will result from the 5B reentering compare to the debris of one of those stages?