I've just opened the OSM site on my PC to have a look at the history in my area, and I've seen this apparently massive edit. It covers most of the US, a lot of Canada, and a significant chunk of Europe and the UK.
wheelmap_visitor says that 'This is a proxy user for all wheelchair tag changes happening on Wheelmap by anonymous users of the website and app Wheelmap.org.' so does this mean that if someone has made edits to areas inside the rectangle at some point recently that they all get included in this edit?
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.
I thought it might be something like that. Thanks for clarifying :)