antenna-theory.com

[Schubert]

Yes, it is because the finite patch above a larger ground plane. Even if they were the same size, you would have fringing fields.

Just think about it. Suppose you had a patch above a ground plane, and suppose there was positive charge on the patch, and the ground below was 0 potential.

The E-field lines must direct directly out from the surface, because there can be no tangential E-field on a good conductor. So directly under the patch, the fields must go straight down.

On the edges, the fields don't abruptly go to zero outside the region of the patch antenna. That is, the E-field exists everywhere due to a point charge. Hence, the E-field points away from the edge of the patch.

Draw pictures and think through it.