How to reduce objects/hands impact on PCB antenna efficiency
First of all, the impedance of your antenna changes due to your hand absorbing the radiation emitted from antenna. You can see this, if you view the Smith Chart display of your VNA. Even the improvement is the body adding capacitance (or/and inductance), thus changing impedance. Therefore, as a rule of thumb, if you reduce the radiation towards the user's hand, or head, they will not affect the antenna impedance as much. 

In regards to your question, I believe the width of the track changes impedance, but it does not make the antenna insensitive per se. Hence I don't think ground loading itself changes the effect of the high permittivity of the human body.

Note that magnetic antennas, such as loops are more insensitive to human proximity, as human body does not absorb H-fields as much as E-fields, and these antennas have predominantly magnetic near fields.