Why does iPhoto '11 use so much memory?

Indeed it did, but this thread dates from a little over four years ago, and you don't mention either the version of iphoto you have nor even the OS you are on.

Wouldn't it be faster to restore from your back up?

Yes if I had one. I recently moved the library to a 1TB external SSD and have not backed it up yet. Time Machine re-imaged a new backup after I removed ~440GB from my primary file system. I've also been suffering from a large NAS share that has been misbehaving with there latest update. Sigh. It will get backed up there.

Still there is the issue of the upgrade. Wish I had a choice on that. I suspect that it wasn't design to handle large libraries. Nor was Apple testing for it. Probably should have started a new thread.

Yes - new threads are best for new problems including the details of what you have and what exactly is happening as well as exact error messages

And note that the iphoto library needs to always be on a volume formatted Mac OS extended (journaled) - this excludes having it on a NAS - what is the format of your external drive?

Not sure what you consider a large library or how you have inside knowledge on what testing Apple did - as a point of information iphoto is good for up to 1,000,000 photos - do you have more than that? Several people here have form 50,00 to 80,000 photos in their libraries - others have from time to time reported more (I think 300,000 is the largest I've seen)

LN