You don't need us to tell you that Scott's style is deeply wordy. This is partly a matter of literary custom: if you look at a bookshelf of 19th century novels like Ivanhoe, you'll see that most of them are enormous. So Scott's wordiness was not at all unusual for his time.
There are other reasons Scott needs to use lots of words, though. Let's take a look at a passage from the beginning of Chapter 43, when he is setting the stage for Rebecca's trial by combat:
Our scene now returns to the exterior of the Castle, or Preceptory, of Templestowe, about the hour when the bloody die was to be cast for the life or death of Rebecca. It was a scene of bustle and life, as if the whole vicinity had poured forth its inhabitants to a village wake, or rural feast. But the earnest desire to look on blood and death, is not peculiar to those dark ages; though in the gladiatorial exercise of single combat and general tourney, they were habituated to the bloody spectacle of brave men falling by each other's hands. (43.1)
This passage is attempting to give us a visual image of Templestowe on the morning of Rebecca's trial. It is "a scene of bustle and life," as the local population is filled with "the earnest desire to look on blood and death." Ivanhoe is a historical novel set in the Middle Ages, and Scott is depicting scenes that are outside the lived experiences of his readers. He has to use a lot of words to give us a more or less realistic sense of what the medieval settings of the novel look like.
At the same time, while Scott uses plenty of words to describe these historical settings, he doesn't want us to feel removed from what we're seeing. Ivanhoe may be set in the past, but Scott frequently makes comparisons between the 1190s and the 1800s so his readers can feel more at home in the world of the novel. For example, this passage ends with the consideration that it is not "peculiar to those dark ages" to want to see "blood and death." In other words, the desire to watch people fight it out continues to this day. By linking his descriptions of the past with his readers' experiences of the present, Scott makes these descriptions more relatable to a contemporary audience.