The latter. There are more than 170 million objects with dimensions up to 1cm, while the larger ones are less than a million.
Other than removed selected really large pieces, such as entire derelict spacecraft, in crowded orbits like GEO where the likelihood of hitting something else is high.
Also, depending on the orbit considered, the debris generated by the harpoon may be less harmful: they have typically a high surface area/weight ratio so they will deorbit fast if the altitude is low enough, like for this mission.