Thanks for the reply re: Maidenpool. All good points, but I am still curious as to why Aegon gave himself such a meagre slice. And I think any way you cut out, the Crownlands are easily the weakest region. There is also a question of timing, given the decision to include Maidenpool in the riverlands. The Mootons bent the knee before the riverlords did, at the same time as the Darklyns. Their new lord even commanded Aegon’s army for a time! Maidenpool would have greatly increased Aegon’s control

I discuss my theories about why Aegon didn’t take more here. The Targaryen monarchy was intended to be a dracocracy, where mere human concerns about land and numbers of soldiers simply didn’t matter. 

Also, keep in mind the necessities of feudal politics. Without large centralized administrations, kings don’t have the capacity to directly rule large areas. Moreover, in feudal societies, the support of the lords is dependent on the king’s generosity at giving out wealth. So trying to hold all the lands yourself works against the king’s own interests.

In this case, his interest in keeping the Riverlands a going concern after a nasty period of being imperial conquests and the battleground of Westeros, when the new overlords were a relatively weak house.