You don’t get a bulk discount for buying a 5000 square foot house instead of a 1200.

Per sqft? You absolutely do. Buying one 5000 sqft house will almost certainly run you less than buying four separate 1200 sqft units, all else being equal. If anything, larger houses are a result of cheaper material inputs. PVC and compressed lumber make manufactured homes easier to produce than their historical brick and mortar and copper pipe counterparts.

So I think you have it backwards. Houses have expanded in size to justify their inflated costs. But the real price inflation is happening at the ground floor.

There is no single cause for houses being more expensive

Privatization and financialization are both but-for drivers of price-inflation. Cheap lending means more dollars are chasing the same number of goods, despite flat wages. The functional extinction of public housing means there's no price ceiling for entry level housing units established by the state. So prices compress around the median purchase rate, while low-income households are entirely priced out of the marketplace and middle-income households are larded up with debt.