Don't use MCN fella, they're frequently wrong (on that little "bike summary" bit anyway!
They are right on both tank sizes, but the hornet has a "spine" style frame - and the tank sits over both sides. Basically means there's about 3/4l in the tank you won't be able to use, so the tank size is effectively about 12/13 on the older hornet and 15/16 on the newer. They're not that efficient either, you really won't get 62mpg on one for certain.
General riding I find them less efficient than the FZS600 but not by much - it's about 45mpg-50mpg around town/commuting gently, 55 ish on long motorway runs (which they're not that good at anyway being a naked bike) and then it'll drop like hell if its thrashed. About 140/150 is probably about right to be honest for combination riding.
My ZX9R & ZX10R under the same riding conditions are almost identical to the hornet I've had and friends have had around town, on a motorway the sports bikes are a few mpg better (guessing more aerodynamic, and rev lower on a constant throtte?)
They are good little nippy bikes, but they're not that efficient. Not sure I'd class it as a huge upgrade over an FZS600 either to be honest, the brakes won't be as good and it doesn't make a massive amount more power - it's just newer, and the suspension is noticably better (particularly if you get the later one with adjustable suspension)