Doubling the width has a greater effect on volume.
The volume of a prism is calculated by this formula:
Prism Volume = Area of Base • Height
The Area of the Base is calculated by this formula:
area = n * side² / (4*tan(180 / # of sides)
So, we see that doubling the width (the side length) will increase the volume by 4 whereas doubling the height only doubles the volume.