An Empire is the domination of a capital city, tribe or nation over others, such that other territories and their inhabitants, are inferior to, or subjugated by, the dominating nation - and are exploited for their resources.
The Islamic Caliphate, however, assimilates all territories as equals to itself. Thus Cordoba rivaled Baghdad and Samarkand rivaled Cairo in terms of wealth, learning and technology. The Islamic lands were all semi-autonomous and prospered as such. The Islamic Caliphate was no more an empire than the European Union is today (however, the EU and a Caliphate are structurally very different, with ultimate political authority residing in the Caliph).
Furthermore, unlike an Empire, the capital cities can change location in a Caliphate - so Constantinople went from being a conquered city, to being the Capital of the Islamic Caliphate itself.
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