Have you ever had a word limit on an essay? It's not because markers are lazy. It's because brevity focuses the exposition. You may have also noticed short *questions* are favoured, as long as they're not so short they don't show effort.

Effort is not in general proportional to length; a long first draft often should be shortened in redrafting before it's posted. (This answer was multiple screens at one point.) As [Blaise Pascal](https://en.wikiquote.org/wiki/Blaise_Pascal#Quotes) said, "I would have written a shorter letter, but I did not have the time."

I usually don't leave long answers, as there's a risk of not seeing the forest for the trees, on part of not only the reader but also the writer. Many long answers do get lots of upvotes, but they have to work hard to announce their structure ahead of time. I see proofs as explanations for why something is true; this perspective allows a short answer to help the OP a lot.