[CHY872]

Probably a few factors.

1. Scaling. You want to reap the rewards of someone else investing billions, and while billions of ICE engines are built every year, most of them are much bigger than 50cc. 2. Tolerances, as you say. I know for example that jet engines have low efficiency at small sizes due to efficiency being driven by the gaps between certain rotating parts, which are relatively larger. 3. Certain parts that need miniaturisation are more expensive on smaller engines. For example, a 50cc would typically have a carburettor, a bigger engine fuel injection. A fuel injector would be significantly larger per unit. 4. Some parts are just harder to miniaturise. For example, small turbochargers have to work harder and at much higher RPMs to achieve the same boost due to area scaling quadratically with diameter.