I like the fleet cap / income tax principle.
It acts as a damping force on the ability to project power (which otherwise would grow exponentially as you expand), and also the ability to replenish losses. This helps the game scale to a wide variety of map sizes effectively. It also somewhat damps out the dominance of a slightly larger empire over a smaller one (or a collection of smaller ones).
However - the fleet cap only scales effectively up to its maximum value. The result is, as highlighted by the OP, that on very large maps several smaller factions may have the same size fleet (each..) as a single much larger faction, leaving the larger empire without a realistic defense should their opponents choose to join forces - even if his empire is physically larger than the combined enemy territory.
The solution to this is to scale the fleet cap with the map size (in terms of # planets). I see two fundamental solutions:
* A fixed, global solution: Scale the fleet cap sizes with the map size. Once the map has been generated, the fleet caps are fixed for everyone. This will allow the largest empires to field reasonable forces on even the largest maps, without altering the current dynamic.
* A dynamic, local solution: Scale the fleet cap with the size of the empire. One example of this would be, increasing each of the fleet caps by a given percentage for every planet gained. Eg, Terran planet +5%, desert +4%, astroid +2% etc. The initial fleet cap could be smaller than the current, reaching the current cap at an empire size of about 20 planets or so.
Anyway, like I said, I do think the fleet cap is good in principle, but it has an inherent limit related to map size and maximum fleet cap.
This is an OBVIOUS fix and I am not sure what the reasoning behind the "one size fits all" INSANITY that prevailed at the publishing point."
ALL though the implementation of this could differ
The graduated penalties as in place:Should be in ratio with the number of available planets and the empire "allegiance" penalties should ALSO scale with the size of the map.
Placing a static Fleet Cap doesn't flex with the research tree decision for resource extraction upgrade.
UNIT SWARMING to bring down someones elses machine would be a problem BUT those individuals shouldn't be playing on such HUGE maps. just play medium sized maps. The game dynamics are elementary enough that there is no bigger fun factor on 100+ planet maps...they just TAKE LONGER TO PLAY.