What I recall was HL2 being a lot easier on the resources compares to Doom 3, which it was competing with. It sparked a community project called DOOM3 CAN DO IT TOO, where they tried to show open areas and water physics. Doom3 itself, being on Mars and using narrow corridors got the reputation that it rendered narrow scenes and got away with being badly optimized. Later, Quake Wars definitely proved the engine was capable of large open spaces (sporting Carmack’s Megatexture technology).

I understood that at first too, but instead of HL1 in HL2 it’s the opposite. It’s showing (something like) HL2 in HL1.