yes it has reason behind, the 2t exhaust is not a simple pipe, it would have lot of maths in it, tuned length,diameter,silencer,
burned gass travel towards the end but there would be some un burned gass with it, which would return back towards cylinder block before closing the exhaust port and then those gass would burn with in coming positive mixter, and that all would depends on exact length of the exhaust, if there would not be a tuned length exhaust then less fuel economy would be in result, so thats why after market big kind of 4t exhaust would not be available in the market