Let's waaay overanalyze this: Courage makes you taller does not mock anyone. It does however acknowledge heightism. It's not mean to say, therefore it's fair game and not "fucked up" the way that the joke about the garden gnome in the OP is. We can make jokes about guys with huge pornstar cocks without that being an insult to those who are less-endowed.

[1] Overanalyzing further: Size-of-hands is fair game, because 1) Trump's hands aren't actually particularly small, and 2) because Trump seems to be sensitive to the subject, it's a comment on his thin-skinned personality not his physical self. Also 3) Trump is a public figure and isn't entitled to the same level of politeness that a private person can expect.