Why Q is the ratio of stored reactive energy to energy dissipated per cycle, and for any resonant circuit, including an antenna, the usable bandwidth is roughly the resonant frequency divided by Q. So raising Q sharpens the resonance and narrows the frequency span over which SWR stays low. Physically short, heavily loaded antennas have small radiation resistance and large reactance, giving high Q and very narrow SWR bandwidth, while fat elements or cages lower Q and broaden the response.
Watch out Increased bandwidth is exactly backwards: Q and bandwidth are inversely related. Gain and common-mode feed line current are separate issues, set by the antenna pattern and by feed line balance or choking, not by Q.
BW = f0 / Q. High Q means a sharp, narrow peak: more Q, less bandwidth.