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I don't necessarily agree with the advice, but I think there's a few reasons to consider it.

A lot of people find their first house too small and buy a larger house later. IMHO, most people have an idea of how many children they'd like to have and some idea of when. I think the idea of the advice is not to buy a small house that fits your needs as a single person and wait until you have a family to buy a larger house that fits your needs as a family, but to just buy the big house?

a) If you buy your first one big, maybe it will be big enough and you won't need to switch. That saves transaction costs, and keeps you price anchored to your first purchase. If your house is in a state with something like Prop 13 that effectively anchors property tax to purchase price, buying the final house earlier can save you a significant amount of property tax over the years.

b) I think house prices rise faster on larger homes than smaller homes (especially condos and things). If that's the case, buying a right size house first then a new right size house later if your needs grow means you'll have a larger gap to cross over time.

c) probably something about house prices always going up, it's implied in a lot of arguments (and it works except when it doesn't ...)