This is a piece from the code of ViewCollection.ascx where this all happens:
Dim UIconfigString As String = GetString("SplitterInitialSizes.Text", Me.LocalResourceFile)
' default start values: 192|589|439|350|439|234|
' tree width, tree height, grid width, grid height, details width, details height
' componentArt goes screwy though and adjusts the actual values.
As you can see it has been a struggle to get it to behave. There is a set of initial values in the resource file under SplitterInitialSizes.Text (ViewCollection.ascx.resx). Try adjusting values here, first. Note also that there are some properties set in the ascx file itself like:
ComponentArt:SplitterPane PaneContentId="TreeViewContent" MinWidth="100" Width="30%" CssClass="SplitterPane"
Here you can see the 30% width parameter.
I think you might have luck playing with these values to adjust it to what you want. If all fails then access to the partial source version would allow you to go in and programmatically change things.