That is completely false. My friend and I just bought brand new 2012's and they have smaller capacity tanks. What's more, the user manual for 2012 (10.2011 4th edition) has changed it's content to include that the fuel quantity is 3.6 gals (13.5L) which reflects what they actually are. The user manuals before that (06.2010 3rd edition) states that it is 16L which is 4.2 gal. I have the 2nd edition, 3rd edition and 4th edition manuals and a new 2012 F800R for reference.
What's more your owners manual did not give the correct capacity as it states it's 16L/4.2 gal since you have the 2011. At least with our 2012's the user manual is correct although the BMW website is still incorrect. The manual also has a disclaimer that says it may not say the right thing.