I don't think the relative size matters. If they are equal mass, they can merge. Probably not likely to have two objects of that sort of mass in close proximity, but if that does happen, there is nothing in any theory that I am aware of that precludes them from merging into a single object.
Equal mass means equal gravitational pulls...so unless they collide they would just orbit each other. While no theory precludes it you have to figure that the chances of them colliding isn’t very likely at all.