When you change the opacity you actually start adding data to the image.
The reason why the JPG is smaller than the PNG is that it does not support transparency and that it's not as lossless as PNG.
Concerning the controlling option please vote for existing idea:
Actually, I am facing the same issue.
I brought a photo into Photoshop (3840x2880, 7.2MB) and made my modifications. The exported files size was a whopping 25.5MB! I then brought this large edited file in to PhotoGene4, made absolutely no edits, and exported the image. The files size is back to 7.2MB
Could you please let us know what photos you were using as well as what you exported them as?
Btw where did you get the image from?
The photo was taken with a GoPro 3. Exported to Camera Roll as JPG.
What I did notice was that when I used Filterstorm Neue, at 100% JPG quality, it also spat out a 25.5MB file. Changing this to 98% yielded 7.5MB. My guess is that Photogene4 automatically selects a JPG quality that matches the original size? I certainly can't tell the difference in 2% quality that's for sure... It's nice to have the option in Filterstorm Neue.
Working on these 3840x2880 files in Photoshop Touch is awful - the images appear grainy and it's slow to use. Pixelmator and others have no problems using layers and working on these size images.