I believe it depends on how many vertices. A polyline with two vertices i.e. start and end point, has two vertices. A line has three: midpoint. There are then, with a polyline sub-entities. This could make them larger, but I don't know. If you've been using autocad since before R13, they had three point plines, just like a line. They were then aware that plines took up more space. This is why they removed the vertice. Never tested it though. Take an empty drawing and test them.
