A poster commented the other day that parents of Maximilians always seem to insist on the whole shebang; you aren't allowed to shorten it. I had to reflect on reading that that it has indeed been my experience of the name. I think that is the sort of attitude people are referring to when they say it is pretentious.
I personally don't really like any of the Max names. "Max" (ditto Magnus and Maximus) just has too much meaning as biggest of all. It just never seems to sit well with the boys I've known, some of whom have been physically tiny and it seems ludicrous, and others who have had kind of overbearing personalities and, again, it isn't flattering.
I'm not sure I really see the appeal.