https://en.wikipedia.org/wiki/Autoped

It's interesting that the engines are roughly the same as the 4 stroke china girl engines you can get for bikes and scooters today, a 155cc and 191cc model.

I wonder if it was a weight/size to power tradeoff, or convention that stuck - was there a targeted engineering reason behind the similarity in size, or have enough things stayed similar in the world of standard parts and sizes that we still have roughly the same engine sizes?

Neat article.