alt Hacker NewsWhen trying to describe sizes and scale, having a reference that one of their readers would be able to go "oh, ... that size, huh?" is useful. There's a meme about "Americans will use anything but the metric system" when describing buildings as "100 school buses back to back" or "3 football fields"; but it's to make numbers real and relatable.
If the article said "500km", someone in the comments would go "that's about as wide as Germany!" or similar.
And, since it was km -> "state size", it was almost certainly for the American audience, since they would have used something like the size of Germany for a European, or "twice the size of South Korea" or something.
For science education, I am strongly for using the units directly to describe everything at first, with these looser comparisons in brackets, rather than inline prose. So I would've instead said, 'is 500 km in diameter (roughly the size of Ohio, or Germany from East to West)'. For extremely large or small values I would use scientific notation only, and expect my audience to understand it, because I'd expect their education to be up to mark.
You said these comparisons are made to make things real or relatable; I personally would expect more out of my audience and give them the magnitude and units directly. I'm not writing for a tabloid; having a strong grounding in units understanding without needing to relate to daily phenomena is in my view critical to not making apparently trivial mistakes that can potentially lead to horrible disasters.