In all my experience from my parent's 1950s built house through all
the houses we've owned the joists have, in the main, been overkill
for the job.

Building regulations recognise that lofts are used for storage and mandate
reasonable joist sizes. Old Victorian houses often have joists sufficient
for holding up the ceiling and a person performing maintenance but little
more. You may be influenced by living in more modern houses that have been
subject to building regs.

Having said this, some modern houses using prefabricated roof trusses seem
to have smaller joists than recommended by building regulations for
freestanding joists, presumably because there is a hanging effect from the
struts.

Christian.