You save money when you buy a container house based on economies of scale not because they’re actually cheaper to build; it's the same for most smaller houses.
It can take a bit to get your head around, but if you measure new building costs based on dollars per square metre, big houses can appear cheaper to build than smaller ones.
Mark Bohan from Genuine Tiny homes says that’s because kitchens and bathrooms are considerably more expensive to build than say a simple corridor or bedroom space, consequently not all square metre costs can be considered the same. You can reduce the overall square metre price by spending more. A four bedroom house may be a hundred thousand dollars more expensive than a two bedroom house, while having a cheaper price per square metre.
"Whether you're building a granny flat or a four bedroom home, both are going to need a kitchen, bathroom, plumbing, drainage and electrics. Building companies can reduce or dilute the costs per square metre by adding more cheap space, whether that is a bigger lounge or more bedrooms."