Sudden reduction in exported jpg size

As others have advised, the PPI will haze zero impact on the size of your JPEG files.

You mention 100% quality for your 30 MB NEF file originally produced 12 MB JPEGs, which suggests that you're exporting at full-size. If the files you describe as being 1MB were also exported at 100% Quality, then this suggests that you've not exported them at full size. So, in addition to @dj_paige suggestion I would be checking to see whether the pixel dimensions match those of the original.

Ths size of a JPG tells you nothing, JPGs can vary greatly in size depending on the original image itself, the amount of cropping done, and many other factors. So, I believe that nothing is wrong here. How can I tell? How can you tell? You LOOK AT the JPG with your own eyes and see if the quality is what you expect, compared to the image in LrC (which you should also look at).