What is the difference between yagi antenna and dipole antenna?

I don't think it has. Bandwidth depends on the diameter to length ratio of the antenna. The greater the diameter of the elements the wider the bandwidth. The inductance goes down and the capacitance goes up, giving the antenna a lower Q. the folded dipole has a greater effective diameter (at least double for the same materials). You can increase a normal dipole's bandwidth by increasing the diameter, hence the old time birdcage aerials.