You probably don't need to worry about the XML/SOAP overhead in terms of message size. A neat tool to see what is actually going over the wire is the TCPMon tool that comes with Axis. Basically, it's a desktop app that acts as an intermediary between the client and the server and lets you view the requests and responses as they happen. It's documentation
is here. (It's written in Java, but it doesn't matter what language the client or the server are in.)