The fact that should be realized is that your 6016 x 4000 pixel images are 72,192,000 bytes size before compression. That is roughly 69 MB, and is 3 bytes of R,G,B image data per pixel. That is simply the size of your RGB image data, 69 MB (uncompressed). That is 6016 x 4000 x 3 bytes per = 72,192,000 bytes of image data in every image. That is simply the size of your image data.
Then JPG is drastic compression, not lossless, meaning, the images do not decompress to be the same quality as before compression. The more compression, the worse they get. Here is an idea at my own site about that problem: https://www.scantips.com/basics09b.html
Your 9 to 12 MB compression is the "normal" reasonable size you ask about. That is already perhaps around 1/8 of the actual data size, which is already a tremendous reduction. That would be the cameras highest quality choice called Fine (Fine JPG Quality, as opposed to less quality). Your camera manual says 11.9 MB is the D3200 average typical file size. (different scenes can compress a little different, but in this ballpark.)
It would not seem reasonable to Not want high quality. Just use your cell phone if you don't want the best. :)
You can make image files smaller by resampling to fewer pixels, but still Fine JPG quality. That is camera Large, Medium, Small, which is just resampling. Resampling to 1/3 size in pixel dimensions is 1/9 size in area, and in bytes. That could be about right for your 1920x1080 video monitor, unless you needed to crop it substantially. Nothing really wrong with that except for printing prints larger than 6x4. But when you end up with the photo of your lifetime, you might wish it larger.
Or you can use extreme JPG compression, and pretty much ruin your images. Both resampling and JPG compresion are irrecoverable, other than going back to retake the picture.
Your 0.5 MB images should show their own faults. I hope you just made copies, and kept the original files too.