Why are the DNGs smaller? I believe it is due to the NEF's embedded jpgs. But Lightroom doesn't really need the jpeg rendering that is stored in the NEF and I could always recreate them later. So how can I drop them?
Exiftool!
+Jeffrey Friedl's blog post at http://regex.info/blog/2006-12-08/303 put me on the right track, but I think his information is out of date now. I found that my NEF had three jpgs:
- JpgFromRaw (Full Size!)
- OtherImage (Fairly large!)
- PreviewImage (Thumbnail-ish)
$ exiftool -list D8H_2754_20131001_183719.NEF | egrep Binary\ data [...] Jpg From Raw : (Binary data 3492514 bytes, use -b option to extract) Other Image : (Binary data 858341 bytes, use -b option to extract) Preview Image : (Binary data 99052 bytes, use -b option to extract) 
I first attempted to just replace the JpgFromRaw with the PreviewImage. That worked, but then I would be just duplicating the jpg -- but here is how you do it:
$ exiftool -v -JpgFromRaw\<PreviewImage 2754_20131001_183719.NEF $ exiftool -v -OtherImage\<PreviewImage 2754_20131001_183719.NEF
So how do I just delete them? Just delete the tags:
$ exiftool -JpgFromRaw= -OtherImage= -overwrite_original_in_place -P 2754_20131001_183719.NEF
(I'm leaving the smallest (~100KB) PreviewImage)
I ran this on a folder of about 11GB of NEFs and when done, the folder was 9.1GB. That's 17% smaller!
I'm going to limit it to this folder for now, but will expand this to other parts of my archives as I gain more confidence that I really don't need the embedded jpgs.
Update: This requires exiftool 9.03 -> Topic: "Otherimage" in NEFs of D800
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