When you save in another format your file, such as a PNG into JPG you can save in a commpression mode for having got a lighter file size. When doing that you cannot see the exact file size resulting.
You can view only the little window displayed but NO resulting file when you chooce the compression level and moreover NO image preview for understanding the right balance between a good image / file size.
Good, i am looking forward. Right now i have to save everything with TIFF, BMP or TGA and after that i have to convert it with XnView to proper file. Is awful when i am creating some web design. But is acceptable even GIMP doesnt have any good options.
Because XnView is Open Source and has a lot of good profesional options, you should consider try their solutions for saving JPEG and take some of their plugins algorythm or just bridge them.