Decloak: That example is rather too extreme to be an accurate reflection, and as such is faulty.
Regiments: 1EP Growth = 10,000 Soldiers, 2 Levels Below. 1EP Training = 2 Levels Up (20,000 Base / 10,000 Quantity = 2 LVLs/EP).
Thousands: 1EP Growth = 5,000 Soldiers, 2 Levels Below. 2EP Growth = 10,000 Soldiers, 2 Levels Below. 1EP Training = 2 Levels Up (see above).
A Regiment nation can do in 2EP what takes a Thousands nation 3EP. This discrepency only grows as numbers go up (4:6, 6:9, so on). The Regiment nation has the definitive advantage from the start. Combined with the fact they also tend to be the richest, a linear Size Stat is unlikely to counter this power, particularly as Size has nothing to do with Growth or Training. From a military standpoint, this capability is unmatched. The counterbalance of slowing down civilian growth is only a minor impediment as odds are, if you're going the military route, you'll just kill people and take their money, making it easier for you to proceed. Plus, Size will affect everyone, not just Regimental nations.
Then again, life is never fair.