Some quick "on top of my head" math tells me ~10% of the population in Spain lives in Madrid or Barcelona, so that pretty much makes it a minority in my head. Put another way, the majority (90%) of the Spanish population lives somewhere else than in Madrid or Barcelona.

Even if you count the greater metropolitan area (where I'm part of the Barcelona one myself, although I live 30 minutes away from the city), you only get up ~20% of the population living in either Madrid or Barcelona. Even then I'd say we're in minority compared to the people who don't live in either those "metropolitan" areas.