I can't believe that with so many people using NG, I found only your post and another one 4 months older without any answer either talking about this issue. Ending up with images way heavier than their should be is not a minor issue especially for slideshow with big images !!! If I compare the image generated by the NG importer and the original one (same size) that I saved in gimp with jpeg 85% quality, the NG one is 5 times bigger (I mean, the file not the image of course) !! In terms of loading time, it's a complete waste.
I played quite a bit with the different options to try to understand the issue better.
- When you're in the image importer, if you uncheck the option "Scale images to max width ..." then you end up with exactly the image that you uploaded. That means that if you use any tool before end to resize your image, you just uncheck this option and you're fine.
- In NG options, in the images tab, you can set the "image quality" option to any value, it doesn't make any difference, the resulting image has always the same file size. And that seems to be the issue, the jpeg exporter is always set to 100% quality. If you try with another tool (I tried with gimp) to resize your image and save it in jpeg with the maximum quality, you end up with more or less the same image size !!