iPhone Gets the size of the original photo

I use the iPhone 4S took a picture, and now with the photos into iPhoto view on a Mac, if that is the size of this picture 4.5M.
Now the program needs to access such devices from the iPhone photo, certainly in the Camera Roll album of this.
I obtained through a series of operations ALAssetsLibrary to such photo ALAssets corresponding object, and then use
ALAssetRepresentation * ref = [asset defaultRepresentation];
UIImage * fullimage = [UIImage imageWithCGImage: [ref fullResolutionImage] \
scale: [ref scale] \
orientation: (UIImageOrientation) [ref orientation]];
The image object is obtained, but this photograph obtained by fullResolutionImage photos much larger than the original, nearly doubled. I checked, iPhoto Exporting photos color sampling is 4:2:2 color obtained through fullResolutionImage sampling is 4:4:4.
I can UIImageJPEGRepresentation for image compression, but this compression ratio is how much would be appropriate?

In fact, I just want to get out of the way 4S original photo shoot with iPhoto out of the same guide that picture. I ask how to do it?