Axy, I didn't post the name of my scooter, since this is the Xciting 250 section of the forum. You are right that I'm a big guy, and I know that this will affect my mileage. But looking at the other mileage comments, I was wondering if there is something amiss with my X 250.
There are actually 3 versions of the Xciting 250 - the carbureted Xciting 250, the Fuel Injected XCiting 250i, and the Fuel Injected with a smaller windscreen Xciting 250Ri. The Ri gets the best mpg - but they should all get better than what you are getting. If you ride in traffic a lot with a lot of stops and starts - then I could see you getting really bad mileage.
I'm 6' 3" 230lbs, and get 50-52mpg riding my 500Ri with a heavy hand. If you are concerned that it's the bike and not the driving conditions then there are a few things you can check.
1) Check your tire pressures. Don't overinflate them - but don't let them run low.
2) Check your parking brake to make sure it isn't dragging.
3) Put the bike on the centerstand and spin each wheel to make sure they spin freely.
Since we're about the same height - have you put handlebar risers on or modified the seat back to move back further? They are both simple and cheap, and made it a lot more comfortable for me (I don't think they changed my mpg though).