It is a case of the "rich getting richer, poor getting poorer" where what is initially a slight advantage swells exponentially very quickly and vice verse. Reynolds continues:
"Anytime where we are doing an historical game we have to make guesses at things... we don't know exactly how something worked, or we're having to model something to put it into the form of a game that you can compress... 5,000 years of history into a few hours of playing time. [..] Try to make these decisions that [avoid this]... further you are ahead, the harder it is for you to stay ahead".
For example, support costs which factor into amount pay for them: when have few citizens, do not cost much to build... as it grows, the more it is going to cost and additionally the longer it is going to take to make each one of them, what Reynolds calls the "bureaucracy effect", where "the larger your empire is, the more expensive it will be to maintain your infrastructure"; while this may sound complicated, what is boils down to is that each step of the way in growing an army becomes harder then the one preceding it... so now, having your army in whole or in part wiped out has a silver lining: you can more quickly re-build it from scratch then what you were having to do to add to it before its decimation.
OPPORTUNITY FOR ADDENDUM
Finally, I would like to take this opportunity to clarify the rules regarding attrition damage in Rise of Nations that I discussed on two previous occasions, first in Part 2 of this series and then again in Part 3, based on some feedback of my Part 3 write-up. Generally speaking, your units take attrition damage only if you are at peace with a nation and enter their territory -- with one exception. Here it is: if you are at war with the ~Russian~ civilization, you still take attrition damage while in their territory vis-à-vis the Russian Winter (this nation's special ability, also previously mentioned in the said parts). I apologize for any misleading -- they were certainly unintentional and, further, I hope that I have now clarified this.
Rise Up: Reynolds On RoN 