Yeah, I have read that Dirt Devils have really odd sizing and that the rears are short. If that is the case, the h-bars would be really hard to turn.

Try putting all 4 on the 650, then measure the rolling distance (distance to roll one revolution). Don't try using standing height or dia. Measure how far one tire rolls per revolution. Use peanut butter, paint, dirty motor oil or a grease marker. Whatever you have to do to measure it reasonable accurately is fine.

Compare the distance rolled by the front tire and the rear. The rear tires should ideally roll 2" further than the front. If you can get it close to that by changing the air pressure, your steering will get easier.