Think about it this way.
Personal licence of Construct 2 comes with multiplayer features. Computers are initially connected to each other via Scirras signal server. Once computers are connected the signal server is not used for anything else.
2 computers are connected to each other via the signal server = peer to peer (although technically one computer will actually be the host/server and the other will be the peer).
2 computers are connected via the signal server to a 3rd computer = set up as a server. What makes this host/server special? Nothing. A server can simply be any computer that is used to launch the game first and that the other players join. A dedicated server on the other hand is just a computer that is dedicated to hosting your game at all times (literally a computer that is always on and running your game through a web page).
Bandwidth is based on how much is happening. Its not to do with the graphics it is about the multiplayer messages and syncing. Are you constantly sending messages to update movement? = More bandwidth.