I like to think about it in terms of output-to-prompt ratio. For HN comments, I think an output ratio of 1 or less is _probably_ fine. Examples:

    - translating (relatively) literally from one language to another would be ~1:1.
    - automatic spelling/grammar correction is ~1:1
    - Using an LLM to help you find a concise way of expressing what you mean, i.e. giving it extra content to help it suggest a way of phrasing something that has the connotation you want, would be <1:1

(expansion is perfectly fine in a coding context -- it often takes way fewer words to express what you want the program to do than the generated code will contain.)

I think all your examples are all perfectly fine.

As for expansion, that might just be the risk we take. I been downvoted on reddit for being "too verbose" in my replies and I'm a human. And perhaps just reading the prompt in that case wouldn't give you more information; the LLM might actually have some insight that is relevant to the conversation. What's the difference between that and googling for something and pasting it in?