why the left column of taskgroup is reduced in customui backstage with excel 2013

Hello everyoneI have a problem in excel 2013 with my additional menu in the bakstage (CustomUI)the left column is not resized

customUI>
<backstage>
<tab id="tabb202401" label="mes tutos sur youtube" firstColumnMinWidth="600">
<firstColumn>
<taskFormGroup id="grtask1">
<category id="truc">
<task id="ButtonTask1" label="Task" imageMso="HappyFace">
<group id="MyTestGroup1" label="Test Group 1">
<topItems>
<labelControl id="Label1" label="Information about this button."/>
<button id="Button1" style="large" label="Click me" imageMso="HappyFace" isDefinitive="true" onAction="TestMe"/>
<labelControl id="Label2" label="Information about this button."/>
<button id="Button2" label="Click me" imageMso="HappyFace" isDefinitive="true" onAction="TestMe2"/>
</topItems>
</group>
</task>
<task id="ButtonTask2" label="Task2" imageMso="HappyFace">
<group id="MyTestGroup2" label="Test Group 2">
<topItems>
<labelControl id="Label3" label="Information about this button."/>
<button id="Button3" style="large" label="Click me" imageMso="HappyFace" isDefinitive="true" onAction="TestMe"/>
<labelControl id="Label4" label="Information about this button."/>
<button id="Button4" label="Click me" imageMso="HappyFace" isDefinitive="true" onAction="TestMe2"/>
</topItems>
</group>
</task>
</category>
</taskFormGroup>
</firstColumn>
</tab>
</backstage>
</customUI>

this is an capture of problem

If anyone has an idea I'm interested