Without dragons, the Targaryens were, on paper, one of the weaker of the Great Houses, relying more on their established seat at the head of the table, which gave them the ability to assemble coalitions against rivals & enemies, than beating them solely on their own muscle. Do you think this is the same dynamic on a lesser scale within the various realms, with Lords Paramount having less incomes & fewer levies than some vassals, using the advantages of Paramount status to maintain supremacy?

To a certain extent, that is ultimately the story of how feudalism dismantled the medieval state post-Charlesmagne, and why kings in many different kingdoms from the High Middle Ages onwards spent so much of their time trying to slowly expand their power vis-a-vis their overmighty vassals. However, it’s not necessarily true that the pattern repeats all the way down – the reason why certain vassals got overmighty is that a lot of lords were good at accumulating as much land as possible while distributing as little as they could get away with while keeping their followers in-hand. 

Indeed, whether king or duke or count or baron, there are two basic rules to medieval politics that follow from the above: first, gain land/power for yourself to remain primus inter pares (while avoiding the level of monopolizing greed that might provoke rebellion), second, divide and rule among your subordinates so that you don’t ever have to fight them all at once and can thus overawe any one rebel vassal. 

And we have some local examples of that: as the Reynes and Tarbecks found out, they did not have more income and more levies than the Lannisters…