Oh really? Yes some modern houses are small, but so were some in 1961. Even new small houses tend to have en-suites, how many people had en-suites in 1961? Some didn't even have inside toilets. The size of the average property today may be smaller but by nowhere near the 26% reduction in household size.
Most modern houses have bigger gardens than the typical yard of a northern terrace.
But they have one kitchen, one lounge, usually one dining room, regardless of how many people live in the house. And modern houses usually have at least 2 toilets.