My patch was 4.2 GB guys, it is mandatory of course so I downloaded it today, I have read that it is 2GB I dont know why mine stated 4.2, I hope that during the installation it jumps at some part but it didnt :P I consumed 4.2 GB from my quota for this and it is visible already on my account stats, the bad part I have to do it from my brother's PC but on the bright side I can copy about 3.6 GB of files from my PC to his and the rest will be downloaded, I read it officially on the battlefield 3 blog that this patch contains the B2K files that's why its big, and even those who didnt buy the DLC will download it (but wont use unless they buy and activate it), so yea eventually I did download the patch (متل الشاطر ).
@AVOlio check this:
http://bf3blog.com/ there is plenty of screenshots, videos and explanations about the Back To Karkand, briefly this DLC contains maps from BF2 that have been redesigned to work with Frostbite 2 and also some BF2 weapons and vehicules.