What's the reason of this result(E_field distribution) in these antennas?

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There are two different antennas shown in the following pictures. The first one has a patch with uniform width and the second one has a patch with non-uniform width.
So the question here is :
Why is the E-field distribution in the second antenna smaller and more uniform than the first antenna?

Is this true? "I think  that the we can interpret this result by this equation:
E=-Gradient(V)
Can we use this equation in this manner :
  E=-Gradient(V)=-Gradient (Z*I)=-(Z*Gradient(I)+I*Gradient(Z))
  But ,what's the continue?"

How can we explain, why is the E-field distribution in the second antenna smaller and more uniform than the first one?
The first antenna:

The second one: