Why does the file size look complete but client says its not?

The file isn't complete until the transfer meter has reached 100%. BitTorrent writes a "dummy" file the same size of the file being downloaded, but its just filled with zeroes.

As the files continues to download, the zeroes will be replaced with the proper data. And in the end you will have a verified exact duplicate of the original file you are downloading.

BitTorrent doesn't download files from start to finish, it grabs what i can from wherever it can, hence the reason it makes a full "dummy" file in the beginning.

You can check out how BitTorrent works for more information on the specifics.