Odd files that Avast seems to see that don't seem visible to the OS

Yes, you are correct – so sorry. It wasn’t a column width problem, it was a mistype on my part because I can’t cut and paste from that “results” dialog, I had to retype it by hand and I am not a great typer.

But I think I am now understanding that a decompression bomb is merely a very very large archive file? Is this true?

Well, not just very large archive file, but rather a big file with a (suspiciously) high compression ratio.