Hi christopher willis,
If you disable the TreeView's Viewstate, you need to add the initial nodes to the TreeView during postbacks too, not only on first load.
The large size is probably not caused by the TreeView's Viewstate, but by the TreeView's new functionality to persist load on demand nodes between postbacks. When a regular postback occurs, the size of the ajax post is reset, but if you prefer you can disable this functionality by setting PersistLoadOnDemandNodes="false"
to the RadTreeView and the ajax post size won't grow when expanding load on demand nodes.
the Telerik team