The reason penis enlargement works is because the tunica albuginea of the corpora cavernosa accommodates the expansion of jelqing by generating new tissue. This occurs over a period of time and with repeated expansion by greater than normal pressure. The glans has no tunica. It is like a balloon. It expands, but rebounds to its original shape. This occurs with internal pressure (jelqing that involves the corpus spongiosum, which is a continuation of the glans) or with external vacuum. The vacuum reduces the atmospheric pressure surrounding the glans and allows your normal blood pressure to expand the tissues beyond the size they would reach with a normal erection. But because the spongy tissue just contracts when the vacuum is released it’s very difficult to attain enlargement.
There are those here who have had some success, but they’ve had to work very hard and usually did so by internal pressure (Uli’s, clamp devices, etc.) and not by pumping. But anything is possible and if you did it often enough you might stretch things out so that the larger appearance would be more or less permanent. Head enlargement is not easy to achieve. You’ll have to work hard if that’s part of your goals.