that not just the case bro, an exhaust on a turbo car works in the same way as on a n/a car. Yes you should have it a bit bigger but too big and you will suffer the effects of not enough back pressure just the same as anyone else. By your theory if you had no exhaust after your turbo it would be quick as and this simple isn't the case. 2.25 is too big for a 1300 turbo or not.
On a turbo car you actually want as bigger exhaust as possible, you'll notice all the turbo drag cars etc don't really run an exhaust.
You don't want any restriction after the turbo, this will restrict flow through the turbine and reduce performance, and I have talked to many people, and putting a bigger dump pipe and exhaust DID make their turbo spool earlier.
It's different on NA cars to turbo cars, when I had no exhaust on my cordia turbo once I put the engine it, it was much faster than when I got the exhaust welded on.