really simple man bigger turbo = increased air intake... = more fuel needed to match air intake so basically the bigger the turbo the more air /gas mixture ur car needs ur o2 sensor might be compensating for the lack of gas i would check that for starters. when i had my car turboed i found that if u put a 300 ohms resistor inline on the signal wire of the o2 sensor actually in ur cat it will decrease the over compensation... dunno just a thought. good luck