According to this study an internegative(described as IN 3rd Generation in diagram) holds roughly 1.5-2k of information.
http://www.efilm.com/publish/2008/05/19/4K%20plus.pdf
This is the reason why Star Wars was subjected to a 1080 x1920 transfer because this was the maximum amount of info that the internegative has.
Hence this explanation from THX engineer Rick Dean(who was in charge of the transfer):
"A technical scan like this allows you to capture the entire body of the image,You're not losing anything, and you're not clipping out whites or losing detail in the blacks, which you'll otherwise never retrieve."
http://business.highbeam.com/3770/article-1G1-122874997/restoring-star-wars-trilogy