Why does the Photos migration process need as much hard drive space (and more!) as my iPhoto library?

I attempted migrating my 661GB iPhoto library (stored on an external drive) to the Photos app tonight. Even though I have 200+ GB of space available on my external drive, I got a Finder message indicating that I need more space to migrate (an estimated 900+ GB based on the message). I'm trying to make sense of this in light of the fact that the Photos app is said not to duplicate or copy images in the iPhoto library. Apple says Photos only builds a reference to the file without copying it. I've read through the various threads on this topic but none seem to offer any conclusive reason as to why this is the case.

Thank you in advance for your time and insight into the matter!