It was basing standards on vehicle footprints. Starting in 2012, a Toyota Corolla had less-strict emissions and fuel economy standards than a compact truck, so they stopped making the compact trucks after 2011.

They brought back the Ranger, but it's larger than some of the older F150s.

And as standards got tighter, it impacted more an more vehicles. All the compact cargo vans (Transit Connect, ProMaster City, and NV200) were discontinued in 2022 because they no longer meet standards at that size.

No, it was the standard that made large vehicles. Not enforcement. And yes smog is a real thing.