So my question is, why use a size 20 2XL? Why not just use a size 16? Seems the only difference would be the hook gap?
I know it was mentioned above, but from later posts it might not have been clear, hook sizes are numbered 1,2,3,4,5,6,7 etc. including the odd numbered sizes, the odd sizes aren't common any more, but, a #16 is four sizes bigger than #20 so substituting this would be going both 4X long and 4X wide gape, as well as heavier wire. #18 would be the 2X size. Not that I think it makes a great deal difference to the fish as long as the resultant fly is the size and silhouette of a common food. I believe that more fish are caught with size 20 flies on a #18 or #16 hook than are caught with what is really a size 16 fly by dimension tied on #20 or #22 hooks, just because of the gape being so small. I see examples of trout anglers telling me they are fishing #22 whatevers and then showing me flies that look like #16s with the point broke off til I look closer and see the point is almost buried in the body, they mention lots of strikes, but missed fish.
As we age our eyes lose the ability to see tiny things clearly, #20s and the tippets that work for them are on my hard to see list and I have not fished any #20 or smaller in years, I think I catch enough trout now on #14 & #16 wets and #16 & #18 dry. (sometimes a short shank helps with proportions)