If (as you say in the second part of your Stormlands analysis) the mercenary armies of Essos are so much smaller than the armies of Westeros, why doesn’t one of the Free Cities try to build an army (standing or not) using the people living in its city and hinterlands? Considering how much more urbanized the western part of Essos is, the people under some of the Free Cities’ control could easily rival at least some of the less populated kingdoms of Westeros.