It just irks me to save a 12 MB DNG file when 2MB will do. ....This results (see jpgs of the DNGs - the forum won't let me post DNGs) only in a reduction from 12MB to 7MB and was lossy.
. First, I don't think you understand what a RAW DNG file is. It is not an image (that's why you can't insert it as an image in the forum) It does not have pixels and is not even RGB. The data block consists of numerical values obtained from each photo site, Each Photosite has a red, or green or blue filter overlaying it. Each photo site is a 12 or 14 bit floating point number that when converted to RGB and combined with the adjacent photo sites can create a 16bit Red, Green or Blue value. It takes all three of these to determine the actual color at a given pixel When you demosiac and convert to RGB, you get a processed pixel at each photo site composite. You also have introduced tone adjustments, fixed the white balance, filtered for noise and sharpened the image. IOW, you have locked in these adjustments that you can't undo without returning to the RAW file and reprocessing.
The other thing that you have in the DNG is a fully processed JPEG thumbnail that the camera produced. This is the image that you see on the camera back screen or when you first open the image in LR.
When you crop an image in LR, you isolate a set of pixels that you want LR you show you from the full image. The only way to reduce the number of Pixels to those in the crop window is to export the image. You can export to a lossless compressed TIFF format but this exported image will always be fixed wrt the processing adjustment applied at conversion to RGB. If you screw this up or want a different result (like correct a blue color cast on your subject), you need to start over with the RAW file.
You are worried about the size of the RAW image file at 12 MB. You should consider the file sizes of full frame DSLRs and Medium format cameras. Mine average about 43MB.
If you have a keeper image, then you want to preserve the original (DNG stands for Digital Negative Graphics), just as you would preserve the original film negative. If you are resisting buying more storage, consider the images that did not make it and
cull rigorously. Most of us have about 5% keepers and the other 95% should be trashed. Yet we are reluctant to discard that 95%. Once you have finished housekeeping on your images, then look at all of the document files stored in your documents folder.