It's not a traffic [bandwidth] problem, really. You have two basic kinds of wireless mesh networks: infrastructure mesh, and ad-hoc mesh. With the former, you can design the network so that each directly-connected node has a dedicated channel and you preserve full bandwidth across the spectrum. You also have only a few gateway nodes so your network updates are very few. With the latter you might be using one channel in half duplex to communicate with whatever nodes are closest to you and propagate joins/parts throughout the network. That will have less bandwidth and be slower to communicate as you add nodes, partly due to number of additional hops.
If the number of nodes who deal with routing increases, that's more nodes that need to be informed of each join/part, so technically it takes longer to update the network. But if the joins/parts are few and the signal is strong, this is a rare event. More nodes can make the whole system faster, or it can make the whole system slower. It depends on the implementation. But there's not a constant flow of mesh routing data that multiplies with nodes; that bandwidth used is tiny.
