3 x wingspan is also roughly the diameter of the loop during my experiment on Low radius loop and
If the loop diameter is lower, so only 2 x wingspan, there is more loss due to the no speed of the chord toward the center, in a similar way as a classical wind rotor.
If the loop diameter is higher, from 4 x wingspan and more, the benefit of the higher swept area is annulated due to both the higher variation of power by the variations of the cosine, and a lesser power far from the central zone of the flight window.