I don't think LLMs have high information density at all, a huge percentage of what they say reads like filler. They're essentially incapable of being concise.

Which I think is partially evidenced by the "tens of thousands of lines" -- I'm working on a media streaming web app for fun on my own time and the whole project is yet to crack 10,000 lines (excluding libraries) including both the backend and the UI. When Claude shows me huge diff at work my first thought is "no way, there's got to be a cleaner simpler way to do this" and there always is

I think it depends on the workflow. I believe I extract a fairly high information density.

That said, this is ultimately a personalized experience, so I think there will be differences between people. Separately from that, I think it's also partly because I basically like writing densely myself. When I start a project, I always use error handling and templates, so I have a fixed form that comes to about 3,000 lines by default.