Tiny engines (ie. sub 20 horsepower) have pretty poor efficiency, and tend not to meet modern emissions requirements (since they haven't been developed with automotive use in mind).
Nobody is putting much R&D into new engine designs.
Lots of countries have laws saying an engine in a car can't be running without a driver present.
For all those reasons, tiny range extenders on large batteries don't tend to exist.
Instead you get moderate or large range extenders paired with smallish batteries (ie. total range 50 miles). And they still have trouble if you drive fast, heavily laden, up a hill, on a hot day for more than the battery capacity.
Nobody is putting much R&D into new engine designs.
Lots of countries have laws saying an engine in a car can't be running without a driver present.
For all those reasons, tiny range extenders on large batteries don't tend to exist.
Instead you get moderate or large range extenders paired with smallish batteries (ie. total range 50 miles). And they still have trouble if you drive fast, heavily laden, up a hill, on a hot day for more than the battery capacity.