Originally posted by justinr I think we are chasing each other in circles here.
Yes indeed jpegs are movable feasts to a certain degree but their
approximate size can be decided upon by setting limits within the compression algorithms which is what we are doing when choosing the jpeg quality level.
That "approximate" file size can vary greatly depending on the amount of detail in the scene. Blowing highlights is one way to reduce detail.
I just took 2 pictures of the exact same thing. Camera is set to 14.2 MP, 4 star quality JPG. Exposure settings are identical for both pictures with the exception of shutter speed.
One is obviously blown, the other is not:
The blown one was 8.2 MB in size. The "good" one was 10.8 MB in size. That's a 25% difference. Even now that they're resized to 800px wide, the blown one is 20KB smaller than the "good" one. Both were re-saved with identical settings.
If the file is 10 MP in size, then it was captured, processed and recorded as a 10 MP file. The camera doesn't fudge a 10 MP file after using fewer pixels.
But again, since we don't have a file with all retained EXIF data, this is all guessing.