The ship types each get a point value based on a fictitious "tier" assigned to each. A Black Widow Golem, for example. is considered a "Light Battleship", whereas most of the other golems are considered "Medium Battleship" and cost about 1.5 as much. Those are still moving around a bit (honestly a Black Widow is more in the Heavy Cruiser realm, but I had to push things up at one point because they were being too painful, I intend to push them back down and adjust point costs and/or the rate of point "income" to compensate).
So it's not really exponential, it just feels that way
The lower fleet ship counts typically means leftover points that are pushed up to the escort ring, yes. Which generally means more starships/guardians. In theory if the points are high enough it can pick anything for an escort, it's just not until you get into things like 300k armadas that it gets silly where individual battle groups have 3 armored golems for _escorts_
One thing to bear in mind is that each city-hub-provoked armada sends one battlegroup at a home command station, one at the refugee outpost (if any, heh), and one at each city hub. So as you add more hubs it will kind of scale down the individual groups a bit, though I'm not sure if that actually makes your turret grids feel any better.
And yea, I'm just removing the refugee-outpost requirement from the shard-built colony ships altogether. There's the off chance of a bit of silliness now (how are you building it without the refugees, etc), but it's not harmful to the fun.
