meaning of melon headed

Imo it's clearly related to illicit drug use. A melon head is outsized and may also be bald.

Incidentally creatures called 'melon heads' can be found in certain US urban legends (Michigan, Ohio, Conneticut).

First theorized in the 1970s, melon heads are said to be completely bald with misshapen bulbous heads, deformed arms and legs, teeth like daggers, and luminous red eyes.

Human growth hormones can affect the size of one's skull. And anabolic steroids can make one bald. Here's an article focussing on the baseball player Barry Bond's changing hat-size due to doping. Extract:

Following the federal indictment of Barry Bonds for perjury and obstruction of justice last week, media reports cited the baseball player’s refusal to “admit that steroids contributed to his swollen head and bloated physique,” and drew a line from his alleged use of performance-enhancing drugs to his “noticeably bigger” skull. Can doping really make your head get bigger?

An interpretation based on some indirect references.

A melon is a large head. Another way to say that is to have a big head is a person with a huge ego or personality. When you hit a home run in baseball, that can give you a "big head". Any major accomplishment can give a person a big head.

So, suddenly melon-headed home-run hitters, are baseball players who suddenly got a big head from hitting home runs and who had probably not distinguished themselves before that.

CUBS PITCHER NAILS GIANTS HITTER RIGHT IN THE MELON

Giants left fielder, Nori Aoki, exited Sunday's game vs. the Chicago Cubs at Wrigley Field after being hit by a pitch in the helmet.

In the top of the third inning, Aoki was facing Jake Arrieta when his 0-1 pitch sailed high and smacked him on the right side of his helmet. Aoki attempted to get out of the way and was still beaned, though his attempt may have caused the pitch to not hit him square in the face.

melon + pitching + baseball

Game of Shadows How Barry Bonds became the melon-head he is today, and why baseball is to blame.

In that article, it means a huge head.

Barry Bonds +melon-head

First off, the prosecutors will run with what we all saw using the ‘ol eye ball test: Barry Bonds’ melon grew to humongous sizes. Recent pictures of Bonds revealed that Baroid has shrunk like a deflated balloon from the days when he was ruling baseball. He went from a skinny kid who could hit to a superhuman baseball cartoon version of the Incredible Hulk, complete with a gigantic melon. Prosecutors are expected to have Mike Murphy, the Giants clubhouse manager, testify that Baroid’s hat size increased during his time in San Francisco — and not because of his increased ego.

baseball + melon

Another way of saying the same thing: He wasn’t being hoodwinked by the gargantuan power of getting a big head from hitting home runs.