Could the noise account for the different in file size?
Exactly. Compression is content-specific, and noise changes the content.
For the 1600, try Noise Ninja or NeatImage and you can get a much cleaner file.
Jeff Spirer
Photos: http://www.spirer.com
One People: http://www.onepeople.com/
Surfaces and Marks: http://www.withoutgrass.com
