As some folks pointed out - a freshly established colony basically has no way to create it's own militia, meaning that it has to either import ground force construction facilities or build them.
I think that it would be better if infantry-based units (no power armor and genetic enhancements... maybe.) can be built locally, while the ground force construction facilities (if we do want to retain them, I personally would prefer to dump them and make fighter factories into arms factories for all the ground forces) can be more 'advanced' facilities - creation of specialized, advanced and armored units, with various special capabilities.
This basically turns them into special facilities, designed to create the finest ground-based formations that your civilization has technology and capability for.
While the remote colonies with some population - they can create basic infantry formations, with little special capabilities (like, if it is a mountainous world, your troops get a 'mountain fighting' capability when trained there).
Back to the original topic:
So the optimal play is to create very small construction formations that you then merge into formations of normal size, and the only cost is your own frustration, time, and hand pain.
Frankly, I'm going to say that the game actually encourages you to create large formations - to get those officers a proper job...
Ever tried dealing with 100 - 500 ton size formations?
They are built fast, yes, but I have only so many officers at start... And considering that the game auto-promotes officers to make a ratio of 1:3, well... You're likely going to not have enough officers, unless you turn on "do not promote".
Again, it is a question of what kind of ground force are you creating - for invasions? Defense of planets? Just as police?
I guess it is valid if you don't plan to get those tiny formations an officer and serve as a reserve or just garrison, but for efficient fighting force - a larger formation is a better option.