Katie Brown's costume

It was the style, at the time which was exaggerated rear-ends (kind of like now, in a different way LOL). Since bustles were considered too fancy and cumbersome for maids, the big bow was a practical alternative.

You'll notice, though, Katie's blue velvet travelling suit does have a bustle, as did the woman on the street in Chicago that Jane noticed. But Katie's "housedress" she wears in the cabin does not. Originally created to prevent heavy fabrics from dragging in the back, it eventually just became fashion for its own sake.

The bustle came into style after the Civil War when the silhouette started to narrow, got very small-to-non-existent in the late 1870s, then came roaring back in the early 1880s. Think back to "Gone With The Wind", when Scarlett and the other women started out wearing huge hoop skirts, but ended up wearing narrower skirts with bustles and fancy hats toward the end.