Jim Lux wrote:
"It`s often explained as "extra capacitance from the bigger size", but I
think that`s not what`s really going on."

Arnold B. Bailey in "TV and Other Receiving Antennas" agrees with Jim.
Bailey writes on page 317:
"We should expect such thin rods to be resonant when their physical
length is slightly less than a free-space half-wave length. When the rod
is thick, the effective velocity along the rod is considerably less than
the free-space velocity, thus reducing the wavelength proportionally."

The above may be grist for Arthur`s mill.

Bailey produces emperical equations (equilibrium?), graphs, and worked
examples for various cross sections.

Best regards, Richard Harrison, KB5WZI