Hmmm, I've never actually checked that. Just had codex run the numbers excluding the boilerplate in some of the documents output is on average ~13% shorter than input, I only checked the ones that were all of the same type (proposals) since it was easier to have codex run the numbers on that.

I compared at final edited output less boilerplate vs sum(prompts).

If I had to guess I'd say far more people are better at editing than they are at writing de novo. My approach probably limits the creativity of the output but I'm not writing a novel.