My recollection is that the battle of Helm's Deep took place over a few pages, whereas the realization of the novel expanded that to (maybe) hours and offered a de novo romantic side-plot with the major character. Nabokov completely upended the narrative frame for the screen-play of Lolita he wrote for Kubrik, but he certainly did not inseminate it with pure pulp.

It is longer in the movies but there are also lots of things in the books that are not in the movie. It makes sense to increase the battle scenes for a movie.

And the romantic plots were in the book as well.

Are you sure you are not mixing up Lord of the Rings and The Hobbit?

Yes, I'm referring to the overtime Aragorn serves as well as the battle mentioned. Proportion is important.