It also has a lot to do with the tolerances built into the original engineering. As stated above, the clutch and gearbox probably has a load limit near 200Nm of torque. The newer Euro 6 engines, however, have been rated at higher power outputs with the same torque figure. All that probably means, however, is that it will still make the maximum allowed torque through a specific rev range (say, 2k to 3.8k as per most small displacement turbocharged diesels) after which the torque curve begins to drop.
Here's the maths bit though. Since power is a function of multiplying force by distance, the same engine will create more power at, say a 4.5k rpm redline if the torque curve drops off less at the top of the rev range without exceeding the maximum 200Nm torque of the running gear. The way manufacturers manage to increase power over generations of the same engine would be to carefully engineer new engine management profiles that would allow for the above scenario (utilizing new injection, ignition and intake/exhaust profiles). This is also the approach taken by reputable tuning companies when they offer extensive tuning for performance purposes, which is also why taking, say an already high-performance sports car to a reputable tuning company for a simple ECU flash costs a great deal of money.
Odds are, however, chip boxes alter engine variables such as injection and boost without taking into account the consequences of when and where the added power comes from, possibly pushing the running gear beyond the design limits. The same can be said for "cheap" ECU flashes. One approach is taking into account the vehicle as a whole system, whereas the other is simply optimizing certain components and values for maximum output without taking into account the impact of all the other variables.
Coming back to the tolerances I started out this post with, as can be seen by the increase in power but not torque in the newer engines, there are still tolerances involved where more power can be extracted from the engine, but it seems the drivetrain tolerances have been met. A good example of such tolerances and how they are met involve Nissan's legendary Skyline series. The RB26DETT straight six engine from the 90's onward and especially in the R34 generation was massively overengineered, allowing for tuners to extract over 500hp with no significant modification to the core engine bits, and over 1000 with a good investment. The gearbox and AWD system was also massively overengineered and was capable of handling the immense power without modification. Come to the R35 generation, however, and "launching" the car too many times would cause massive damage to the dual clutch transmission, also voiding the warranty. The debacle was great enough that there's a whole business around building strengthened R35 Skyline gearboxes, even for the stock, untuned engine.
Edit: It seems all small Fiats with the 1.3 Multijet series get the C150 transmission, which is rated to a maximum torque input of 210Nm. On the petrol side, the only gearbox rated for over 200Nm of torque input appears to be the Abarth gearbox (C510).
