I tried some new things:
I opened the same file (20MB) and ran the blur filter 7 times and saved again as jpg, and the file size dropped to 800 KB,
I opened that file and ran the unsharp mask filter on it several times and that 800 KM file climbed to 8.5 MB.
I opened that 8.5 MB file and ran unsharp mask on it serveral times, and it climbed to 20 MB
Opened that 20 MB file and ran the blur filter several times and the file size went down to 700 KB
You are observing normal behavior.
JPG artifacts are introduced with each save. As artifacts grow in number and intensity, the file size grows.
When you apply unsharp mask, you increase the details in the image data (larger file). When you blur, you reduce the image data (smaller file).
If you repeatedly edit images, you should be using a lossless format.
PS: I also doubt that your camera is saving to Photoshop's level of 12. It is probably creating the file around what Photoshop would measure at 8-10 on its scale.
If you open a jpeg in PS and save in quality of 8, 10, 12 you will see quite a difference in size of file.
Then open the original, and the 3 versions and look at 600%. See if saving at 12 is any better than 10 or less at a smaller size.
Your right Don!
I went to another Mac running 10.5.8 that I own with CS4 on it .
Sam thing happened. I was surprised how the file size climbed with each application of unsharp mask, but with blur or different other filters, the file size was reduced to nothing.
It was interesting to note that there is a limit to file growth using the "Unsharp mask".
After 20 applications of the unsharp filter, the picture quit sharpening (having reached it maximum radius, it could apply no more sharpening as the edges in the photo showed no more growth.... I guess) and the file size quit climbing at around 24 MB.
I thought maybe it was CS5 and OS Lion, but the same thing happens in OS Leopard and CS4.
Thanks for confirming my results.