Image dims when viewing in full screen in iphoto on thunderbolt monitor
I am seeing the same thing here with a MBA on the TB display. All fine on MBA display, and thumbnails on TB, but once you zoom up the photos on the TB display they become 'darker'. 

If I export the photos out of iPhoto and display them in Preview/quicklook, they don't go darker. It is only iPhoto that is rendering the images with some colour profile. 

I couldn't find an answer but a restart of the computer fixed the issue. 

Using iPhoto 9.5.1 running OS 10.9.4 on MacBookAir5,2.