Size of identical image when exported at "Maximum" quality JPEG: 6.9MB
So why did the size of this JPEG image double when it was exported?
There’s only one possible explanation. In order to do the export iPhoto is first decompressing the image, then re-compressing it using a 99% or near-lossless JPEG algorithm. This doubles the image size while degrading image quality. (You can’t improve the quality of a JPEG image by recompressing it.)
In older versions of iPhoto the app simply copied the image. Now iPhoto behaves like Aperture.
I suspect this back-asswords behavior is related to a surprise of a few months back, when people who thought they were archiving their images to MobileMe learned that Apple's "full resolution" was in fact a high-quality JPEG (about 97% compression). iPhoto was presumably doing a similar decompression/compression cycle on upload to MobileMe. (Supposedly this was "fixed". I'm skeptical.)
Apple's heading for a fall.
Update: If you export as "Current" you avoid the perverse "maximal quality" degradation. Apple should have made "maximal quality" JPEG the same as "current" for images that are already JPEG.