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:
Madrid vs
Barcelona
Glasgow vs
Edinburgh
Los Angeles vs
San Fransisco
Damascus vs
Aleppo
Moscow vs
Saint Petersburg
Sydney vs
Melbourne
Calcutta vs
Mumbai
Toronto vs
Montreal
Nairobi vs
Mombasa
Sao Paulo vs
Rio de Janerio
Zurich vs
Geneva
Johannesburg vs
Cape Town
Ho Chi Minh City vs
Hanoi
Milan vs
Rome
Of course, New York City, Paris and London are the irreconcilable exceptions. And heaps others.