I think this isn't quite straightforward, as there are two objects to consider here: the 'subwindow' control; and the 'windowclass' rendered in the subwindow.
The subwindow control is basically a placeholder, which defines the location of a subwindow and its class.
The rendered window class is stretched to fill the subwindow control.
I think, therefore, that you need to change the subwindow anchoring/bounds and not the window class placement etc.
For examples of this in the d20 ruleset, take a look at adventure_npcs.xml. There is an ultimate 'parent' window class (called 'npc') and a window class for each tab (for example 'npc_combat'), with a subwindow control (in this case 'combat') which binds the 'npc_combat' window class to the 'npc' window class.
The size of 'npc_combat' is given by the 'combat' subwindow control, not by anything on 'npc_combat' itself.
This window class contains the layout for combat data
<windowclass name="npc_combat">
<datasource>npc</datasource>
<script>
...
</script>
<sheetdata>
...
</sheetdata>
</windowclass>
This window class contains the layout for the npc in total
<windowclass name="npc">
...
<sheetdata>
...
This subwindow control binds the npc_combat window
class to the npc window class. It dictates the size of
npc_combat.
<subwindow name="combat">
<anchored>
<to>tabcontentframe</to>
<position>over</position>
<offset>-15,-7</offset>
</anchored>
<class>npc_combat</class>
</subwindow>
...
</sheetdata>
</windowclass>
I hope that helps.
Stuart