Small pipe wont allow your turbo to spool up as fast as larger pipe. Turbos rely on pressure differences in the exhaust system. A turbo creates backressure between the engine and the exhaust inlet of the turbo, without this the compressor will not spin. The greater the pressure difference between the inlet of the exhaust side and outlet of the exhaust side of the turbo allows the turbo to spin up to speed faster. The smaller the outlet pipe of a turbo, the more backpressure on the exhaust outlet, which equals less total pressure difference across the exhaust side, which equals slower the spool time and lower outputs.