[AngelicaDeAlquezar]

I don't think it's the etymology what makes one word in Spanish sound fancier than other, but I'm unable to say what actually determines that a word is fancy or not. One just doesn't hear such a word very often, either because it's archaic or because it "belongs" to the realm of educated writing.

Of course, the words derived from Greek or Latin are more unusual in English, whose foundations come from different places, but those are the basic etymologies of most Spanish words.
When I write or speak in English, I tend to use the direct translation of many words that are very common in Spanish, and English native speakers take them for fancy words; only then I learn they're not as common in English and that I may even not be understood because of that.
In any case, it's just by learning and practicing the language that one gets to know what words will be closer to your listener or your reader.