I just imported 32GB of images (my local Images folder) from the past 10 years into iPhoto. I was honestly surprised that the result was an iPhoto library of 65GB. I have no RAW images and am surprised that the space is double what I expect. But on a SSD, this is really taking too much space.
One of the explanations though, was that the library consists of iPod/iPhone cache files, preview files and thumbnail files. Those folder minus 65GB results in still 51GB of images.
I extendedly compared the local folder with the 'show contents' folder from the iPhoto library and I couldn't really find any differences, but the file size of the folders themselves..
So who knows how to reduce those 65GB to a regular 32GB or can give a decent explanation..