The rectangle is the bounding box for all the edits in that changeset. What I'm guessing happens here is that wheelmap.org bundles a bunch of random edits from global users into a single changeset and then submits it. Since its users are from random places around the globe, it's likely the bounding boxes are going to be large.
But just because the bounding box is physically large doesn't mean the edits are large. For example, a single edit near Los Angeles coupled with a single edit near Copenhagen would produce the bounding box in your screenshot.