Thanks for the reply.
You should be able to reproduce the problem by doing what I suggested:
1. Compress a JPG in a photo editing program to 80.
2. Upload the image with watermarking on.
3. Download the image and open it in a photo editing program and check the compression level. It should show 66 instead of 80.
Do the same with an image that is uploaded without watermarking - it will show less compression.
The picture links that I sent privately show a lower quality on the smaller image size. That's not just my opinion, it is proven by the fact that the compression level went from 80 to 66 after watermarking.
I would like to figure out how I can make the compression 80 after watermarking instead of 66.