I've written previously that iPhoto '08 is Aperture-Lite, but with nastier bugs. Apple appears to use a lot of the same source code for both now.
So I decided to retest export behavior in iPhoto and see if it was still smarter than Aperture.
In particular, Aperture is very dumb when managing an export of mixed JPEG and RAW files. If you select JPEG format rather than current format, Aperture doesn't simply export the original JPEG, it processes the JPEG and then repeats the lossy compression.
iPhoto used to be smarter. If a JPEG original was untouched, and you specified export as JPEG, iPhoto would export the original JPEG. Perfect.
The good news is iPhoto is still smarter than Aperture. If you specify export as JPEG on a mixture of RAW and JPEG images with "maximum quality", iPhoto will export the JPEGs without transformation.
EXCEPT, if you embed metadata like rating and title. In that case iPhoto expands the original and writes a new file that's been recompressed.
So a 1.8 MB unedited JPEG original becomes a 4MB export if the metadata feature has been used. A bigger file with quality loss due to repeat lossy compression.
It doesn't have to be this way. GraphicConverter and other apps will allow EXIF header changes without a lossy compression cycle.
So iPhoto's JPEG handling isn't quite as stupid as Aperture's, but it's headed that way.