Why An antenna element behaves like a resonant circuit, and bandwidth depends on its Q: lower Q means a broader response. Making the elements fatter lowers the conductor's inductance-to-capacitance ratio, which lowers Q and widens the SWR and gain bandwidth. That is why wide-band Yagis and log periodics use thick tubing, cages, or multiple parallel wires per element.
Watch out Squeezing the elements closer together does the opposite: tight spacing raises Q, which can raise gain on one frequency but makes the feedpoint impedance and pattern change very rapidly with frequency. Loading coils electrically shorten an element and also narrow the bandwidth; tapered tubing is a mechanical and weight compromise, not a bandwidth booster.
Fat elements, fat bandwidth. Low Q equals wide; skinny elements and loading coils equal narrow.