Why does the Dropbox Backup site show folder size as ">...." instead of the actual size?

Thanks very much Jay for you quick reaction. Just to clarify:

When you say it's possible that Dropbox backup doesn't calculate the size of of all subfolders, is that a known issue from other users and Dropbox is working on fixing it or is it just your assumption. I guess I find it a bit unsettling not knowing the exact size of a folder. With that I feel unsure that all my files have been backed up and I am going to want to go in check all the folders and subfolders to make sure they are all there which quite inefficient. 

Noted on the file size differences, thanks for the explanation.

Thanks in advance,

Florence