To clarify my opinion... I am not opposed to a slight buff to Portugal province base. But balancing its armies around that of Spain because Portugal did well in wars vs Spain and retained its entire territory throughout the course of the period is not the proper way to go. EU4 has no way to distinguish between offensive and defensive/liberation wars ; countries have standing armies with relatively static force limits, this has been discussed a million times - there is no emergency levies/draft, or morale boost that could help the defender. Giving a country the strength of its neighbor because it historically survived against said neighbor is not how things are handled : historical survival without expansion is usually depicted in game with increased coring cost, relations buffs and other deterrent to invasion. This kinda sucks but that's how it is.
Within the current mechanics of the game, we cannot have a Spain that's capable of beating France while losing against Portugal, unless we want Portugal to be consistently tougher than France. Does anyone want that ? And we cannot have a Portugal strong enough to resist Spanish expansionism without having a Portugal strong enough to more often than not expand within Spain, especially considering that, given the current rival mechanics and diplomacy setup, that Portugal would likely ally with Aragon or France to polandize its yellow neighbor.
Finally, if we judge how well a nation is balanced according to its historical achievements, then Portugal has one of the better balances ; it's even slightly OP. A successful Portugal retains its territorial integrity and becomes a colonial and trade powerhouse, and such is the outcome of most of my games. Portugal is usually even richer than it was, and has a bigger empire. A trade and colonization nerf for it isn't a disaster. There are other nations that, by such standards, are way more underpowered. Does AI Russia consistently colonize Siberia and conquers big chunks of Lithuania/Commonwealth while completely destroying the Hordes ? Not even close. Does AI Great Britain becomes a dominant colonial power with the richest Asian trade companies ? Usually not. Even the Ottomans are relatively tame compared to their historical counterparts. That's why I get slightly annoyed when I see recurrent complaints - obviously fueled more by nationalist sentiments than by concerns for gameplay - about Portugal being underpowered and in need of a substantial buff.