So, I've made a similar thread to this from a while, but it got ignored...
The reason I made another one is that I talked with Support and my lovely Hyperion was the one replying. He said the following:
1) " The main difference is that the Shadow ones don't fold up as tightly in the folded position, so they take up a little more space." - Hyperion
2) "While the position is slightly different, we don't see any really noticeable size difference" - Hyperion
3) "It's also worth pointing out that the ones you named as bigger have kind of an aura around them that extends past the edge of the wing itself; that can create the illusion of greater size as well." - Hyperion
And when I told him that "Why don't you make them the same?" He told me to make this suggestion on the forums, and here I am. Please, it may be not that visible, but to me, I want the wings to look larger. In fact, I bought them because I wanted a shiny, visible wings to show-off (That's what accessories for, right?)
Maybe Hyperion can come here and support me. ;3
Well, I can definitely say that the size is not an illusion, those wings are indeed larger than the rest. What with the Prismatic wings being in the same tier of rarity as Volcanic and Divine I would expect it to have the same sizes and the like, but they don't. And that disappoints me.
+1 for bringing Prismatic Dragon/Valkyrie Wings on-par with the Volcanic, Divine and Shadow variant sizes.