The idea that in a country or territory, the second largest city is better than the largest. Better in terms of quality of life, but also to some extents being the place with the atmosphere you as a tourist would prefer to soak up.
This is probably because in history large cities attract industrial development, foreign invaders, grandiose government monuments, yuppies and over-population. The second largest city however avoids this unnecessary attention, and thus is left to develop at a more steady pace and without cosmopolitanism diluting its identity. It could be because the second largest cities were built around beautiful bays or mountain ranges (like Cape Town, Rio de Janerio or San Fransisco) that later became natural obstacles to urban growth.
Consider which city in each pair is a nicer place to visit:
vs San Fransisco
vs Saint Petersburg
vs Rio de Janerio
vs Cape Town
Ho Chi Minh City
Of course, New York City, Paris and London are the irreconcilable exceptions. And heaps others.