I think I just answered my own question
I was using the Save item under the File menu rather than the folder icons in the main app window. Under the Save folder icon I now see there's a somewhat cryptic list of options -- MatchXXX (256, 512, 1024) and Original. If I select Original this seems to preserve the original resolution. It would be helpful, I suppose, if these options were visible when Saving from the top menubar as well, but no matter -- this is free s'ware, a gift to us all, and I'm suitably grateful now that I understand how to preserve the original resolution!
Many thanks to the author and contributors. This is faster and more intuitive than InkScape's 'Trace Bitmap' feature; the result is a bitmap rather than a set of paths, so scaling is somewhat limited (I may still use InkScape where I need to scale up), but it's so much faster that xnSketch will now be my preferred tool for this purpose.