Thanks for the comments.
In the end I put
ini_set('memory_limit','512M');
in the sitemapxml.php file called by the cron, to keep it self contained.
I just thought I'd post this to save others wondering why the two approaches (admin/cron) may have different results. It took me too long to work it out.
I don't need to call the 7.3 binary explicitly. I just left it in there as I was using 7.1 for this until I started investigating it recently and wanted to keep the command intact for reference.
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