Well for one if its a jap spec S15 it has a steel wheeled T28 turbo, so it is larger than the stock RB25 turbo, this is why they make more power on a stock turbo.
As far as turbo vs turbo wise goes on different engines. A turbo will max out at a certain power regardless on what displacement engine it's on. You have to think of airflow wise of the turbo, not so much the capacity of the engine.
The larger displacement engine will make more torque from it's displacement and will spool the turbo up quicker due to having more exhaust gas. However once the turbo reaches it's max effeciency the game is over power wise.