Again, I was lucky. I found David Jefferies through his website and got him interested. He, being a Ph.D, and a Professor of Electronic Engineering in the UK could explain the phenomena that appeared anomalous to me. The thicker the wire the greater the loop perimeter. The greater the perimeter the higher the loop impedance, the lower the losses, the higher the gain and the lower the Q factor.
People normally construct Yagis out of thick aluminum tubing.
They normally construct quads out of thin copper wire. Then they expect the quad to outperform the Yagi. Rod elements, in Yagis, do not. As David Jefferies has explained, the coupling is via the corner currents and the non-radiating wires which connect the radiators. The lesson we learned from this is that the ideal element spacing in quads has to be far wider than published in the old, stale, literature.