The new Diesel technology is forcing OEMs to install a Selective Reduction Catalyst to reduce the NOx pollution and to do so they need a Urea (aka DEF Diesel Exhaust Fluid) tank. For a SUV it is about 10 gallons. This tank is filled every 10,000 miles and the fuel tank is probably made smaller to accomodate the DEF tank. A standard fuel tank is about 15/25 gallons.
The "Urea in tank" acts like a balloon inside the fuel tank. The results are that the total volume of the diesel tank is improved and every drop of urea consumed gives more room to diesel at the next diesel fill up (this occur every some 5-600 miles). The range will increase as the Urea gets consumed. One Cons is that in order to fill the whole urea tank the diesel tank should not be full.
As an example inside a 25 gallon diesel tank, there could be a 10 gallon urea tank. The diesel tank capacity would then go from 15 to 25 gallons as the consumer drives the 10 thousand miles between DEF refills.