One thing I've been wondering about is whether the iCloud Photo Library is the same size, or smaller, than an iPhoto/Aperture Library containing the same images. Has anyone who's uploaded their library made that comparison?
My thinking is that Apple wouldn't want to keep Master, Preview, and Thumbnail versions of each image up in the cloud (they are still present on Mac). Thumbnails might be needed for the same reason they always have (faster display of images when browsing - in this case, browsing iCloud.com), but I see no real benefit to keeping previews. Considering I have 5.1 GB in my iPhone's Photo Library (Settings > General > Usage > iCloud: Manage Storage > Photos & Camera) and 4.35 GB in iCloud (iCloud.com > Settings), it suggests they're not keeping thumbnails in the cloud (or are using lossless compression of some sort).