In general you can say that the Communities (Gemeenschappen) mainly have powers that relate to people (education, culture, welfare, youth, sport, ...) while the Regions (Gewesten) mainly have powers that relate to a territory (spatial planning, public works, public transport,...).

While that is true, you can not seriously claim that maintaining a forest has the same cost associated with it than the 2nd largest harbor of Europe, and one of the most dense road and rail networks in the world.

Infrastructure cost is more related to population size + economic activity, than to surface area.

To give an extreme example:

The infrastructure spending of New York City (8.6 million citizens, 784km2, 1550 billion USD GDP) is about 11 billion dollars a year (those numbers are for the city, not the state!).

The least population dense state is North Dakota (0.8 million citizens, 183000km2, 28 billion USD GDP), and they generally spend less than 1 billion a year on infrastructure.