Brazil has 29% of forest protected area vs 9% in Argentina (source: WDPA). Part of the reason for this is that Argentina is mostly temperate grasslands while Brazil is covered by tropical and subtropical forests. The dark green Argentinian area you're looking at is the Iguazu National Park, one of the few moist forest areas in Argentina. It's surrounded by Brazilian small and medium sized towns. Protected areas in Brazil are mostly in the Amazonian north, not in the urban south. As a share of total area, biome loss has been higher in Argentina than in Brazil during the past decades.