Why At RF, the impedance of a bonding conductor is dominated by its inductance, not its DC resistance, and inductance drops as the conductor gets wider and flatter. Current also flows only on the surface because of skin effect, so a wide flat strap gives much more usable surface than a round wire of the same weight. Flat copper strap, kept as short and straight as possible, therefore gives the lowest impedance path to ground for bonding station equipment and lightning protection.
Watch out Shield braid stripped from coax looks tempting because it is flat-ish copper, but its woven strands corrode, break and behave inductively, so it is not recommended for permanent bonding; copper-clad steel and twisted pair are round, high-inductance conductors.
At RF think surface, not wire gauge: short, wide, flat strap wins.