Probably both. Due to zoning in almost all American cities it is now illegal to build houses that are as small as in the post war period.

Houses are much bigger today.

Because of complete NIMBY victory, not because the youth are greedy. YIMBY is the way.

Maybe partly. But developers make more money building the biggest houses they can sell. So that's what they build.

True, YIMBY can only do so much. The market serves wealth-weighted people not people-weighted people, so when inequality cooks to the point that the whims of the wealthy outweigh the needs of the poor, the latter gets the boot when the two are in conflict.