Best is to post a image of structure (I don't have HFSS). In fact the RF source is in between two pieces of metal. In a dipole case, each piece of metal has same size. In your case, one side is called "monopole" and the other side is called "ground plane". I see the ground plane more as a counterpoise.
The impedance as seen by your souce is the sum of the monopole impedance and ground plane impedance (roughly spoken). When the monopole is in quarter wave resonance (low-Z) and the ground plane is more or less in half wave resonance (high-Z), the impedance as seen by the source is dominated by the ground plane impedance as the ground plane impedance is the highest of the two. If so, the ground plane provides most of the radiation.
This is not just a theoretical issue, but can be seen in practical antennas, especially in case of ground planes with large length/width ratio.