Cyberpunk 2077 is the most direct urban-open-world comparison for GTA 6. Different design philosophy though.
Cyberpunk's approach
Tall, narrow city. Dense verticality - skyscrapers, megabuildings, layered districts. Total area smaller than GTA's. The streets feel infinite because of the verticality, not the spread.
GTA 6's approach
Wider, more horizontal city. Vice City is dense at street level but doesn't stack the way Night City does. More spread, less stacking.
Trailer evidence suggests pedestrian and vehicle density at GTA 6's street level may exceed Cyberpunk's.