Proportional representation is great when the population is fairly homogeneous with similar political issues and goals, and you don't need a union of different independent regions and groups. The European Union would not work with proportional representation, and countries like Iceland would struggle with their very large difference between city population and country population.

The point of disproportional representation is to reduce the political power imbalance between of high population areas with low population areas. The alternative is to split up the country or union so that each unique area and demographic govern themselves in isolation. Few people want that outcome so a compromise has to be chosen. In iceland as an example, the combined country districts has a majority if they all band together, while the Reykjavík district is by size the absolute single biggest and has well above 50% of Iceland population. Its a compromise that while political power is still very much centered in Reykjavík, they can't blindly ignore the country population.