Why is a motorcycle's (150 cc or below) service interval recommended once every 4000 kms or 4 months where as a car's is once a year or 10,000 km.
Isn't a car with more power in need of more frequent service than a small engine displacement bike?
Service intervals are more of a function of tolerances and MTBF than anything else. There's extensive data available for passenger car longevity, not so much for motorcycles, so car manufacturers likely just have more information at their disposal. Also, motorcycle engines rev to speeds close to double their passenger car cousins and RPM increases internal engine stress exponentially (way more than, say, doubling the HP does).
For smaller vehicles/engines the manufacturer may also just be erring on the side of wastefulness rather than do any long-term studies of reliability.
Valve Clearance Tolerances are Tighter on Small Displacement Engines
As an example;
Valve lash clearance for a 1982 Honda XR200 is 0.002in on the intake and 0.003in on the exhaust
Valve lash clearance for a 1986 Nissan Z28 engine is .008in exhaust and .009in for the intake
The Z28 engine has much larger valve faces as well as a considerable longer valve stem which, when heated, will increase in length. Tolerances for the larger valves are considerably larger. There is more 'wiggle room', if you will.
When/if the valve faces begin to 'seat' over time they will consume valve clearance and have a tendency to hang open a bit if the lash is consumed by the valve face beating into the seat. You can see this condition as a groove in the valve face. Lash typically needs to be added and the frequency of service is more frequent due to the reduced lash due the ICE being more susceptible to the condition of hanging open. Primarily due to the lack of lash in the tiny snowflake of an ice that Honda cited above has.
Basically, less lash in the tiny motor forces a higher frequency on the maintenance schedule.