Imported 32GB of images into iPhoto, 65GB library is the result - Huh? 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..
First thing to know : There is no possibility to reduce the size of an iPhoto library without corrupting it. 

What you can do is explore your iPhoto library in ~/Picture/iPhoto Library => right click => show package content. 

In this directory you'll see multiple directories : 


Masters : holds in the original of your pictures
Preview : holds in a low resolution preview of your pictures
Thumbnails : holds in thumbnails like the ones used on faces. 


These 3 directories are the main reason of the weight of your library. The other files/directories are way lighter.