Well, what kind of data do we need to send?
-Client contacts server to join game.
-Server allows client to join game.
-Client sends key and mouse input.
-Server sends positions of players, physical objects, projectiles, health info, etc.
-Client or server disconnects.
-Game score, map changes, etc.
For example, the server might send ten "messages" in one packet, to update all the player positions. I am pretty sure it would be better to send all ten players' data in one packet than to send it in ten separate packets.
My feeling is the number of packets will be much more of a bottleneck than the actual size of the data.