Philip Rhoades writes:
> What still doesn't make sense is that if the original file is JPG and 
> one simply opens it and then saves it as another JPG file with 100% 

Because JPEG isn't meant to be saved at 100% quality.

The JPEG FAQ,, says:

  Except for experimental purposes, never go above about Q 95; using Q 100
  will produce a file two or three times as large as Q 95, but of hardly any
  better quality.  Q 100 is a mathematical limit rather than a useful setting.
  If you see a file made with Q 100, it's a pretty sure sign that the maker
  didn't know what he/she was doing.

Do a web search on
  jpeg quality "100%"
and you'll find lots of detailed discussions of this.

GIMP's "Show preview in image window" check box is extremely
helpful, and lets you see the trade-off in quality versus size.
It's too bad it's not enabled by default.

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