Better than Kiki, not as good as Laputa... (I've seen so many Miyazaki films now I'm getting the urge to rank them) This one is also built around Miyasaki's enthusiasm for flight. And also, maybe, Casablanca? With the jazz bar full of criminals and the protagonist full of inner demons. An enigmatic, cursed, noble but fallen hero, an anarchist giving his life to fight off fascists. A Byron, except of course it's not Byron, "It's mine! See you later!"
The most striking scene in the film is the bedtime story in which Porco describes his near death experience and transformation. Reversed when, as with the frog prince, he is kissed by a woman who loves him. The link between the two events is Gina, it seems. Her first husband was Porco's comrade, now dead. Perhaps Porco's swinish form is a symbol for survivor's guilt, which fades when he receives an unequivocal sign that Gina has moved on. There is a certain logic to that interpretation, tho it feels a bit pat, and I'm pretty sure Miyazaki had deeper things on his mind.
My guess would be that it's a gender thing again. Boys will be boys going off to fight stupid wars, marrying sweethearts and getting themselves killed. Porco survives all that disfigured, convinced of the end of innocence, the darkness of man's heart. And the women, Fio, Gina, are there to redeem the world. By showing that they are just as resourceful as men, and kinder too.