I also have this question. I've been deleting apps and music all day to try to get this to update with no success. Why in the world don't they have the "size" of the download in the update notes? Come on guys get with the program!
Does anyone know the answer to this? I have only tried to update on my iphone, i haven't tried my ipad yet.
The 1.2.1 version of the app itself is 801.3 Mb. Since apps appear to be essentially compressed folders with a .ipa extension I imagine that gets unpacked into a larger space on the device. Only when the files have been successfully decompressed and checked to be complete can the previous version of the app, and the downloaded file be removed to free up space. If it says 1.9 Gb isn't enough then give it some more to play with, at least until the app is updated.