1) The "ball of doom": Like in Warcraft 3, the fixed number of units could force you to always keep your entire army in a tight formation. Two fronts, distraction maneuvers, or a navy not involved into the current battle would immediately mean defeat.
I played WC3, know what you are talking about, and am guessing it will -not- be a problem in ciV.
This happened in WC3 because the cost per unit was
not a smooth function of the number of units. When you jumped past 50 food, you harvested only 7 per trip instead of 10, a discrete jump in costs that made people hover at 50 until they had a good reason to jump.
However, to my knowledge and based on the approach firaxis has taken in other aspects of the game, I think per unit costs will be a
smooth increasing function of the number of units.
If you think about it, optimizing with a discontinuous cost function often results in corner solutions (it is optimal to stay at 50 food for a while), whereas optimizing with a continuous cost function yields interior solutions (the exact size of optimal army depends on the value of available objectives.. exactly what you are looking for I think).
To be specific, I think we'll have, and correct me if I'm wrong:
1) A hard cap on a the number of a specific kind of unit (e.g. swordsman) per resource controlled (e.g. iron).
2) Some as-yet unknown cost of making new units that prevents huge armies. My guess: a per unit maintenance cost that is an increasing function of the number of units you have. The first few may be free (as in cIV), the next few may cost 1gpt (as in cIV), but after a while each new unit may cost 2gpt, then 3gpt, etc (similar to how each new city in cIV costs more than the previous one).
Together these things make an fairly smooth increasing cost of each new unit I was talking about. Sure you can reach a point where you can't make more swordsmen, but you can always make more archers or catapults.. it will just become really expensive to do so eventually.
Finally, I agree it will probably be optimal to have most (90%) of your army on the front, and the rest holding down key resources or perhaps in reserve in case of a sneak attack from another direction. Sounds about like how real armies were deployed, its not like we had a big garrison in chicago during WW2.
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tl;dr = ciV not like WC3 because there are smooth costs instead hard food caps.