The Battle of the Pelennor Fields is a crescendo structured in four/five stages, depending on if you want to count in the Grond or not. It we count it in, we have this progression: one, the siege begins with its regular siege stuff going on; two, the Grond makes its appearance and breaks the main gates of Minas Tirith; three, the Rohirrim come and kick the ass of every orc that gets in their way; four, the Haradrim join the war with their Mûmakil and crash the Rohirrim as if they were ants; five, the king returns with his dead-ly (hehe) army and they shatter every living being on the fields.
In this situation, you can't just break the structure and make the following step weaker than, or equal to, the previous one, because in both cases you'd be breaking the progression and that would also be anticlimactic, but even worse, because the audience would be left with a bittersweet sensation of the battle not being as good as it could've been. Thus, what you could conclude that you should do is making an over-the-top climax, but the problem with that in this particular case is that you can't finish this battle with a too high note, because this is not the final battle, even it feels like it is. So ending this battle with too much epicity would leave the audience already satiated with greatness (so to speak), and the consequences of that would be 1) that the final battle would feel less important or even completely redundant, and 2) that the whole conclusion of the movie would be excessively saturated with artificial epicity, which would harm the movie more than help it.
However, if you give the audience an anticlimax without breaking the progression of the battle, you leave the impression on them that it was good and epic, while still leaving them eager for more and not fully saciated. So as long as that anticlimax leads to something actually bigger, that's not a problem at all, because you're more than going to make up for it. And that's, of course, the case with The Return of the King and the Battle of the Black Gate, which is better as a climax than the Battle of the Pelennor Fields because of Aragorn's speechall the tension generated by the convergence of the two big plots of the movie: the one that takes place in the realms of men and the one featuring Frodo, Sam and my precious.
And that's also one of the reasons why The Return of the King is so praised as a conclusion of the trilogy (apart from just being an excelent film): because the lesser time you leave between the close of one plot and the close of the others (and between the first plot closed and the end of the movie), the more satisfied the audience tends to feel. And The Return of the King closes its two main plots at the same freaking time.