This game has an unusually high amount (usually over 15 kbyte/s) of it for a 4 player, non-peer2peer game, and surprisingly, I don't think it's caused by players because in town the usage is quite low. In fact, the only time I've ever seen this type of upload usage is being the host in a peer2peer game.

Not only does this mean some players will experience some connection issues (upload bandwidth for most people is much more limited to download bandwidth), but it also means you'll be paying more for server hosting in general.

So, maybe this is something you can't really do much about, but if it is, I suggest you take a look at optimising it.