Re-downloading the entire game sounds odd.

Some patches can be big though and Steam always automatically patches your game, so if it has been a while since you played, it's possible that it's just downloading a few (huge) patches, which make it seem as if it's downloading the entire thing.

Also, very recently, a new patch has been released (see : http://forums.totalwar.com/showthrea...-April-2012%29); I think it's about 130 MB or so, so if you're not on broadband or Steam is just being slow again, and you have a few of these to download, it may take a while.