Its to do with how much fuel the ecu gives the motor, this is effected by how much air it has to mix it with and what timing as well, if you have a small (stock) turbo the standard ecu is set to give fuel steadily untill boost comes on, then it loads up more fuel because of the extra air being pumped in, it is set at certain points in the map and corrected by the air flow meter.
With a bigger turbo the whole map is different because boost comes on at a completely different point.
You have 2 choices really, get a stock turbo or a ecu and tune.