Consequences of the coalition agreement for home builders?

What was the reason for the large storage for the small photovoltaic area?

I live in Belgium, where the situation for photovoltaics is much less favorable for private households than in Germany. Additionally, thanks to subsidies, the larger battery cost only 1000 euros more than the 5kWh one. I also factor in the wear and tear of the battery. After 20 years, the originally 10kWh battery will probably have only 7kWh capacity left.
So it turned out that I chose the somewhat too large storage capacity.

I live in Belgium where the situation regarding photovoltaics is much less favorable for private households than in Germany

Different countries, different electricity prices, different subsidies. In Belgium, electricity is also very expensive, almost at the German level. If you are satisfied with the solution, you have already done the most important thing right.