Remember Ancient Greece, it was the relatively small size of the city states that enabled them to have a special sense of belonging to an atomic (not divisible) group, and flourish in it. Now take massive states such as the mongol empire; as soon as their thirst for keeping expanding was ended, they collapsed or became very backward entities, which is true for the ottoman empire as well.
Those empires collapsed because they were simply too big for the government to exercise effective sovereignty over them. If you think how heavily consolidated the Roman Empire was - with its road networks, military garrisons and governors with direct links to the Emperor, for example - and how often it fragmented in its early stages, it becomes apparent that empires such as the Macedonian and Mongol never had a chance. The city-states are just modern nations in minature - even then they had the concept of hellas as demonstrated almost every time Greece was invaded by some external power. Improvements to communications enabled the Greeks to feel like one nation by the time of Ottoman rule. The same happened in England, which was a mess of feuding tribes when Caesar found the place and has since merged into one ethnic group - the English are just 'English' and bear no more basic national identity.