When you receive an Add activity into the inbox for a paged collection you don't control, how do you know whether or not to split off a new page in that collection? It doesn't seem like Collections know their own page size...
@jdormit I think that you shouldn't really allow activity submissions on a paged collection. In my opinion the client/server adding the activity should dereference the parent collection IRI (from the partOf property) and submit it there.