When comparing harvest volumes or tax numbers between different parts of the Roman Empire it’s important to remember how hard it can be to get good comparisons. Rome lasted a long time and from one era to another the relative importance of different areas could change greatly from century to century. This could be due to climate change or tax policies or different divisions and of land and revenue between official state figures and private figures.
Another enormous variable was population.
Unlike the modern era it was quite common in that time for even prosperous agricultural regions to have fallow or poorly utilized land simply due to lack of labor to work it all. This can easily be seen in the way the Romans and others of the era went to great lengths to get and keep people in agriculturally productive areas; granting soldiers freeholds, acquiring vast numbers of slave laborers, accounts of crashes in harvests after regional epidemics - there often simply weren’t enough hands to go around and do all the work needed to get maximum productivity in all places at all times.
This is before accounting for different weights or standards of coinage in different parts of the empire or at different times, and vastly different levels of administrative efficiency in different places. Part of what made Egypt so useful to the Romans was the way that agriculture there depended on a collective effort to maintain the irrigation and drainage systems to control the Nile floods. To do it efficiently ‘built in’ an oppprtunity for the central state to keep tabs on what was happening and efficiently collect taxes and grain surpluses. Many other areas might have had equally good harvests at times, but administratively would be more costly and difficult to efficiently collect grain and taxes from.
