The Han dynasty built the Weiyang Palace , the Tang Dynasty built the Daming Palace and the Ming dynasty built the Forbidden City that was then used by the Qing .
On a global scale ,there always existed an empire of similar size to these dynasties : the Roman Empire for the Han , the Abbasid Caliphate for the Tang and the Mughal empire for the Qing .
Yet these other empires never build something of such a scale ;the Imperial Palace on the Palatine Hill ,the Palace of the Golden Gate and Khuld Palace of Baghdad and the Red Fort of Delhi were all smaller.
Why made the Chinese willing to build such giant structures and how were they able to build them when no one else could ?
EDIT : Thanks u/IconicJester, the Red Fort is in fact bigger than the Forbidden city so this comparison does not make sense .
I'm not able to answer why the Weiyang Palace was so much larger than later Chinese imperial palace complexes, but I can shed some light on why Chinese palaces were so large in general.
First, what exactly constitutes a "palace" (宮 gong) in the Chinese imperial tradition? It included, of course, the residence of the reigning emperor and his immediate family, but it was much more than that. In addition to the emperor's own lodgings and the halls where he attended to state business, there were separate quarters for the imperial women (the empress had her own while the consorts shared another) and for the eunuchs and maids who attended to them. But the palace was also major a cultic site, where the emperors conducted most of the religious ceremonies associated with their office, calling for specialized altars and platforms. Lastly, they also included the offices, archives and courts for a large part of the imperial bureaucracy and for the palace guard. For a large and top-heavy administrative state like Han, this meant a staff of thousands working in the imperial ministries for law, finance, public works, the capital garrison and other departments, plus the metropolitan and provincial administration for the capital and its densely-populated environs.
Access to the palace was tightly policed, so that it could only be entered with express authorization, and was further internally divided so that access to the emperor's person (or to the women of his harem) were controlled even more tightly. So, in practice, the 宮 gong was less what we conventionally think of as a "ruler's palace" and more like an entire government district, closed to the ordinary populace and housing the great bulk of the Han central administration. This is why the Weiyang Palace was a vast complex of more than a dozen separate halls and pavilions with specialized functions- it was not just a luxurious residence but also the center of imperial administration and religious rites.
Can you clarify what you mean about the Red Fort being smaller than the Forbidden city? I know little enough about them, but I've at least seen the Red Fort, and it seemed enormous. A quick google suggests that the Red Fort is the larger of the two, about 180 acres vs. 250 acres. Am I missing some nuance of measurement?