Hi,
I’m glad you are using (and happy with the app). Sorting by size is useful feature, but the problem is that Photos doesn’t provide that information in an efficient way. Basically, I would have to download every original (if it’s in the cloud) and get the size at that point. That’s why a lot of information is only available in Edit mode or after the image has been downloaded – Photos provides only the bare minimum (they don’t even tell me whether the original file is a RAW or not).