Return loss of Dual Frequency Antenna

can anyone tell me why the return loss for first resonance is high as compared to the return loss for second resonance?

The first thing that comes to my mind is higher losses at higher frequencies. So you get a lower S11.

It depends on your design, your circuit is tuned for first resonance that is why you see big dip in S11.
You can make your circuit more tuned @ 2nd resonance also.
I can tell this because my resonating structure was by mistake got tuned to 2nd resonance.