Most nuclear fission products don't remain radioactive for long periods either, let alone "virtually forever." Bear in mind that the longer-lived a radioisotope is, the less radioactive it is.

For JET, I dug up the actual numbers (LLW is low-level waste and ILW is intermediate-level waste):

Radioactive wastes arising from operation and decommissioning of the JET experimental nuclear fusion reactor, located at Culham, are already factored into the UK radioactive waste inventory. Forecast LLW and ILW packaged volumes are 4,120 m^3 and 480 m^3, respectively; activated steels and alloy plant and equipment, including the JET vacuum vessel, are a major contributor to the ILW arising.

According to this page a typical 1-gigawatt fission reactor produces 3 m^3 of high-level waste per year, 7 m^3 of intermediate-level waste, and 90 m^3 of low-level waste per year while operating.

I'm having trouble finding easily comparable numbers for the wastes produced during decommissioning, this page had a lot of detail but was focused more on the area of land that needed to be sealed off rather than the cubic meters of material contained there. It does talk about the mass of some of the turbines being considered as low-level waste being in the range of a few hundred tons, which isn't much.

It's true that spent fission fuel rods are high level waste, but the total volume of that is quite small and it's in a very manageable form. So overall, I'm not really sure there's going to be a big improvement on nuclear waste production with fusion power. It's certainly not going to be a panacea, we're still going to need nuclear waste repositories and still be dealing with processing and sequestering large amounts of materials there.