As a conductor's diameter increases, what is the effect on its electrical length?
Why Electrical length is measured in wavelengths or degrees, not feet, so it depends on how fast the wave travels along the conductor. A fatter conductor has more distributed capacitance to its surroundings and a greater end effect, which slows propagation (lowers the velocity factor), so each foot of metal represents more electrical degrees. That is why a thick tubing dipole must be cut physically shorter than a thin wire dipole to resonate on the same frequency.
Watch out The choice that says it decreases confuses electrical length with the physical length needed for resonance: the physical cut gets shorter precisely because the electrical length per foot went up. Saying diameter has no effect ignores the well known shortening factor used when designing fat element beams.
Fat elements get cut short, because fat means electrically long.