What does VW do to improve HP and Torque from their 1.4 TSI engine?
CAXA: 90 KW @ 5000 200 NM @ 1500-4000
CAVD 118 KW @ 5800 240 NM @ 1500-4500
Both engines you mentioned have a supercharger and turbo combination. The performance difference is achieved by increasing the boost pressure to the engine. While this might be done only by using different ECU maps for small performance differences, the big performance differences usually require higher capacity parts as well, especially for the turbos, injectors and cooling.
A short google search revealed the following for these engines:
90 to 96 kW variants — two-part plastic intake manifold, turbocharger incorporated in exhaust manifold with maximum boost pressure 1.8 bars (26.1 psi), water-cooled intercooler integrated into intake manifold
103 to 125 kW variants — multi-ribbed belt-driven fifth-generation Eaton Roots-type positive displacement supercharger operated by a magnetic clutch integrated in a module inside the water pump, internal step-down ratio on the input end of the synchronisation gear pair, and KKK turbocharger with integrated wastegate connected in series, administrated by a control flap, 2.5 bars (36.3 psi) pressure at 1,500 rpm, front-mounted intercooler (FMIC)
Not to forget different fuel specs:
90 to 103 kW variants — 95 RON ultra-low sulphur unleaded petrol (ULSP)
110 to 125 kW variants — 98 RON 'Super Unleaded' ultra-low sulphur unleaded petrol (ULSP) - 95 RON may be used, but will result in lower power output
The differing specs of VW engines are (I believe) mainly done by varying the ECU maps - Which is why a lot of power can be gained from the lower-spec engines by chipping them up to (and beyond) the higher-spec versions.