I'm going to have to disagree, big words, or 'pretentious' writing do not make purple prose: the overabundance of words themselves make it purple prose. A sentence of "he was elated" is not as prosey as "he was so happy he could jump the sky and high five all the green men upon the Martian surface."
I've never understood the great distaste for a well informed word. My fear in regards to this so called pretentious writing is that to avoid pretention, I don't want to devolve into writing 'See Spot. See spot run!'
Perhaps this is an unrealistic concern, but it perturbs me greatly.
Well, I agree that a lot of it does have to do with overabundance of such words, but there's also another part to consider. This other part is why would you say that someone is "exultant" instead of just elated? There's a difference between simplifying speech and using words you find in the dictionary to "spice up" your writing (read: make it sound more intellectual), and that's a problem a lot of newer writers tend to have. Like I said, the more academic you get (i.e., the more you move towards words that would make your work sound more like an academic discussion than a fanfiction) and the more pretentious you get, the more ridiculous you seem to other writers because it comes off as you trying to sound like you're intellectual. (There's an abundance of people in fandom who believe that the key to being a genius is sesquipedalian loquaciousness. And it gets awkward dealing with them, let alone trying to read their work, only to realize that if you take away the flowery language, the story doesn't really go anywhere and have overdramatic, brooding characters. That kind of writing alone gets associated with the kind of flowery language that we're talking about on a frequent level just because it happens so often. It doesn't really help that the narration itself ends up sounding unnatural because of the author's language choice. So in a sense, it's like script fic in that it has negative notions attached to its form, not so much because the individual work is bad but instead because it happens so frequently that a reader will be more likely to brace themselves than swallow what you're saying, unfortunately enough.) That isn't to say you need to use the same words over and over again. In fact, "elated" is a perfectly reasonable word to use. However, there's definitely a difference between that and a number of its lengthier synonyms, and there's a difference between what is and isn't going to be considered ridiculous and/or awkward.
Of course, using a plethora of them in a short span is going to make things worse for you, but if you have the choice between describing someone as overjoyed and describing someone as ravished or rhapsodic, you may want to consider just going with overjoyed. Doubly so if the synonym you choose has horrendous alternate meanings. (Hello, ravished.)
Point is, part of it is how many times you use flowery language. You're absolutely right in saying that. But there's also a bit of weight on which words you use, so to speak. Avoiding pretentiousness doesn't necessarily involve reducing your prose to basic words. It actually more involves writing what sounds natural to you. For a lot of people, academic/pretentious words don't sound natural (because "no one talks like that" -- as in, no one actually uses the words we're talking about in everyday language), and that's just one of the reasons why readers get so uncomfortable with seeing them pop up.