It could, but that would mean an added delay before a file can start transferring, which is something most people don't want and is likely to actually be useful rarely, purely to keep a status display up to date. Of course my point was that the file size could potentially change at any time during the transfer too, so there are so many times it's possible to get out of sync, the fact is that most of the time it's pointless checking.
Note that your file transfer will only complete when the server sends a transfer complete message. The size is most just for display purposes.
