I'm guessing that the driven belt pulley - the one on top of the cutting disk - is larger diameter than the one on the engine crankshaft. If it's not, the slasher is badly designed. The idea is to get about the same blade-tip speed as for an 18" mower, with a cutting disk that is one third bigger. The belt drive is used as a way to get the required blade speed, when the engine's maximum horsepower speed is too high for direct drive to work properly. Also importantly, when you hit the odd log or water pipe, the drive belt will just slip - the engine's crankshaft won't be broken by the impact.
It is usual for slashers to have belt drives. They do not waste much power - their efficiency is around 90% - and they enable you to use a smaller, cheaper engine and at the same time avoid engine damage from impacts. You could use a big, low speed engine running at the same speed as the cutting disk, but it would be more expensive and heavier. It would probably be a better solution for a machine to be used commercially, but for a light duty slasher, a cheap Victa engine is a good way to go.
