> I would think volume is a better metric because landfills don't really much care about mass.
Most EU countries have actually outlawed landfills in favor of waste-to-energy systems. [1] Of course waste-to-energy only works for stuff like plastics, not for electronics which you'd rather want to disassemble carefully to recover the precious metals.
> If you wanted to save trash you'd probably go after packaging.
Which the EU does too. There is legislation underway (or possibly already passed) to outlaw many types of single-use plastic items. Everyone switching from plastic straws to paper straws may actually be another case of Brussels effect.
> it's so far down the scale as to be pointless to prioritize over almost anything
The thing is, from experience we know that these legislations take so long to enact and enforce that you cannot just start with the biggest items, wait for it to be done, and then move to the next biggest item. You need to tackle many possible avenues for waste reduction at once.
[1] https://www.cewep.eu/landfill-taxes-and-restrictions/