Why do you consider the Roman, Persian, British, Caliphate and Mongol as a major empires?

The Roman, Persian, Mongol, and British Empires, and the Caliphates (Rashidun, Umayyad, Abassid, Fatimid, Almohad, and Ottoman) can be considered great for their geographical and economic conquests. All of these Empires managed to conquer and incorporate great expanses of land into their territory and in most cases, exploit the local resources and economies. 

Many of these Empires had long lifespans where there territories were in flux as land was conquered and re-conquered by opposing forces. What was really distinctive about these Empires was their capability for establishing uniformity throughout the land. For example, Roman trade networks ensured that the goods and money being used even in the most distant provinces were of Roman style.The Sassanians (a Persian Empire) were able to establish a uniform revival of orthodox Zoroastrianism, and the Mongolian Empire even had a postal system connecting the lands.

These empires can be considered driving forces in cultural change for much of the world over the past 2,500 years. Their power can still be felt today in the language, architecture, art, religion, and customs of Europe, Asia, Africa, and the Americas.