Your original post said that your output file was 600MB. If that's correct, it's a gigantic file, which may be breaking some internal limits of the Kindle or calibre. By reducing the number of articles fetched from each feed, if you can get the file down to a more reasonable size, my sense is that it will work.
By way of comparison, a typical Sunday NYTimes download purchased from Amazon, with 200+ articles, is ~3MB.
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