Oooh that is a sweet looking scooter. It looks like the tires are lower profile than stock, it's possible the overall diameter is no bigger than the 12" tires with a taller sidewall. I don't like low profile tires. Not only do they give you a rough ride due to the fact that the sidewalls don't flex as much, they are also more likely to allow wheel damage. A lot of cars I see around here with 24" wheels and tires with a 1" wide sidewall have rim damage to the wheels.
2002 Vulcan 750 (now being slowly reassembled) 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch moped