Larger values give larger bulbs. Negative values give a bulb on the other side of the plane. When zero, the bulb is like a sphere that just touches the plane. When positive, there is a crossover point. When negative the bulb simply bulges out of the plane like a pimple.
Higher values make the top end of the bulb fatter.
The height of the spike is always about 1 unit. The 2d version of the Witch of Agnesi curve can be extruded in the Z direction or rotated about various axes by use of the SOR parameters. Controls the height of the spike. To produce a surface that matches the 3d version, the height parameter needs to be the square of the size parameter. This time I've cut it in half so that you can see the way that the rear face puckers forward to meet the dimple from the front face.