Why do I need such thick wires in household electrical installation?
However, as the existing installation and googling show, it is a common practice to use 2.5 mm2 or thicker wires for everything but lighting, and 1.5 mm2 for lighting. I wonder why? Aren't these standards outdated?


Home wiring has been standardized over the last 100 years. More and more loads are being added to receptacle circuits while lighting is actually becoming more efficient. This is why the lighting is normally done with smaller wire. This is reflected in your minimum size breaker at 16 amps for 1.5 mm2 wire. Ours here in the US is a 15 amp breaker with a #14 wire. 


  Obviously for modern LED lighting, even 1 mm2 is ridiculously thick, based on such calculation.


However, this is sized for the entire circuit not for individual loads. So, since LED lighting does draw so much less power it just means you can now wire a house with many more lighting outlets on a single circuit. This frees up more circuits for the appliances that modern humans loves. 

There is a method to the madness and if you spend enough time in the industry you will overcome the learning curve.

Have fun in your garden!