Why so big?

[carasdad]

Why so big? Getting ready to buy an Adly Thunderbike 50 from my ex boss..who I was a Scoot mechanic for. It has 470 original miles. Fastest stock 50cc 2T I have ever ridden. Which confuses me since it's Her Chee made...but still your standard Minarelli engine. 55+mph out of the box by only adding a Technigas pipe. Rest is stock. So I assume the Minarelli 70 BBK's should really turn the power on. My question is why is the belt so big?? Long Case Minarelli's use a 788 belt...the spare he is tossing in is 878 and what the lil booklet calls for. So why would the Her Chee Long Case engine use a longer belt than it's twin...the Long Case Minarelli?