After doing some more digging, I've figured out a usable solution to this problem which I will post here for reference.
The problem is that the Graph object does need to be larger than the size it started at in order for you to zoom in. However, we only want to see a portion of that zoomed in image at a time, corresponding to the initial size of the Graph. To
do this in WPF we can use clipping (see http://www.alksolutions.com/en/Blog/Can-Balioglu/Clipping-in-WPF for a good explanation) on a container object that holds the NodeXL control. Grids work well for this, as do a few other Panel-type classes (Border,
Canvas, etc). The XAML should look like this:
<Grid x:Name="ClipGrid" Width="500" Height="500" ClipToBounds="True">
<my:NodeXLControl Name="theNXLcontrol" Width="500" Height="500" />
Setting the ClipToBounds property on the Grid gives the exact behavior we want - the Graph can still zoom in and expand, but the visible Graph is clipped by the dimensions set in ClipGrid.
Hopefully this will be helpful to others who have had the same issue.