Jpeg Quality is the measure of how much Compression is used on the file.
The settings on your camera don't matter much in this case, because it's not that file being exported.
When you export, iPhoto puts your Photo into a new file and you can choose to set the compression level on this file. So, you have chosen to have the smallest level of compression. That's why your files are larger.
The difference is the level of compression, as to which is more appropriate: depends on your intended use.
I think I read from OT that high keeps file size the same size, medium keeps half the file size. (I know that is not compression).
If an expert like you wanted to export your photos for safe keeping (backup), would you use medium, high or maximum? So the intended use is safe keeping and backup.
What would you experts do?
Thee is very little correlation between the file size on the file created by your camera and the one created by iPhoto on export. Just forget about that. It's of almost zero relevance.
So, the fact that x or y setting produces a file about the same size as the one created by your camera means... nothing. It's a different file, created with different settings, and - if you have edited the shot, added metadata or whatever - it has different contents.
So, the question is... what am I going to do with this file? Upload it to a website keep the file small. Archive for posterity? I would use the largest file size - leaves you most to work with.
Personally I do not backup photos - I backup my iPhoto library - with Time Machine hourly and daily with Carbon Copy Cloner plus an occasional off site backup - that gives me full flexibility on what to do and quality since I have everything
If I were going to bakcup just part of the information, the photos, I would use the maximum quality