@Doranq Great posts, very well thought out. May take a second read to fully grasp all you said.
The viewpoints expressed so far fall mainly into two categories: (1) Why is it thought to be okay to dis small penises? (or more broadly, is it okay?), and (2) aspects relating to the article that prompted the question and the politics involved.
I'll skip the 2nd for now. It's been discussed already. It tends to devolve into a political debate and does not really address the question of ridiculing small penises. As far as I know, I don't think either party has taken a stand on penis size, although it was briefly brought up during the Republican primaries by Messieurs Trump and Rubio via the proxy issue of tiny hands. So far there has been no similar outcry over the possible harm this might bring to people so manually afflicted. And, although it's no excuse for the imagery, no one's really suggesting that the intent of the perpetrator of the prank was to ridicule small penises, but, rather, a political candidate.
I'll also skip the second part of (1): Is ridiculing small penises okay? and weigh in on the first part, why is it perceived to be okay?
And for that, I have no easy answer. There's much to be said about the suggestion that it's been left unchallenged, that small-cocked guys are in an awkward place to defend themselves or others. As a guy who's hung like a chipmunk myself, I've not really been subject to that much of this type of assault, and also face the dilemma of confronting the issue. To do so is, like the closeted gay, to "out" myself as being in the penially challenged crowd.
(Er. . . not to suggest closeted gays are penially challenged.) It's difficult to come up with a slogan for the small penis protest march which wouldn't lead to further ridicule. And how many un-penially challenged men would march with them. "So you've got a small todger too, Jim?" [Snicker]
There's probably something to the suggestion that it gives those outside the subject group an opportunity to puff themselves up and improve their image among others. An age-old tactic: Build yourself up by tearing others down.
When the humor is directed at a particular individual, it's obviously a great way to strike a blow (below the belt) against someone, one not easy to respond to. As a penially challenged man, I can attest that there'd be no easy comeback to this. To respond about one's success with women doesn't really cut the mustard. Most anything you say just digs the hole deeper. Your best bet would be to join in the laughter, as if it doesn't matter.
Why do people not frown on this like they do on other cruel types of statements? Perhaps just because it's disguised as humor. Cruel things said by Joan Rivers and Don Rickles about others go unchallenged because it's considered humor. Did Liz Taylor really think it was funny to have her weight problems continually lampooned on television by Joan Rivers? In real life, women may laugh it off but I'm sure it hurts inside. Polish or Italian or Jewish or black or blonde jokes have all been "acceptable" at one time or another. Or not. Even the mentally disabled have been fair game. ("Why did the moron. . . ?") In these cases, the excuse may be that "obviously" no ill intent was meant towards the targeted groups. Do people really think that Polish are really dumb? Of course it may not be so innocent as that all the time. A few of those groups have been considered by the ignorant to be so. And the intent of the jokes were not always benign. But the smaller-endowed don't seem to be the object of hate groups, but more simply of plain old cruel and sometimes thoughtless humor.
I think the simplest answer is probably the one advanced here a few times. Many do consciously or subconsciously consider smaller-hung men to be less "manly" by virtue of the size of their penis and therefore open to ridicule. We associate "manliness" with big penis, and lack of manliness with just the opposite.
A mind test along these lines: How often have you heard a story that goes something like this? "I saw this big, muscular football-player type guy in the showers. Real macho type guy. . . but he had a real tiny penis." What's the subtext? The key is the word "but". Everything preceding the word "but" is the classic depiction of masculinity ("big", "muscular", "football", "macho"). The "but" signals what follows is just the opposite, i.e., not masculine: "tiny penis". Hence, smh;; penis is not masculine, or, if you will, feminine.
In our society "un-msculine" men are deemed funny, an oxymoron. (How often have we seen effeminate gays the subject of humor, also?)
As the target of public humor in movies and on television, certain groups or types of people are often dehumanized. We don't consider how this may hurt them. We don't empathize with them. Or, even to the extent that they're seen as individuals, as somehow worthy of being the butt of jokes. "They deserve it." We probably wouldn't tell these jokes in front of a person we know has a small cock, unless he somehow indicated his acceptance of the humor, or there was animus involved.
So I guess I'll finish with the conclusion that the reason why society finds it okay to make fun of guys with small cocks is because we somehow find smaller-hung guys as not being felly masculine and therefore somehow inherently funny.