Why should phase difference be small when resonance is small?
do you mean why should the phase difference be small when the forcing frequency and natural frequency are the same ?
Here is (in thumbnail) a swept-frequency plot of the voltage across a parallel RLC circuit. A constant current source (1 mA) is used. The resonant frequency is 1000 Hz. The voltage amplitude (solid line) is on the left. The phase (dotted line) is on the right. The phase difference (between voltage and current) equals zero when the forcing frequency and the natural frequency are the same.
Separate names with a comma.