Using the Minimum Width for CollectionView

I have a CollectionView which I want to use the minimum necessary width. The containing Grid has the following ColumnDefinitions:
<Grid.ColumnDefinitions><ColumnDefinition Width="Auto"/><ColumnDefinition Width="*"/></Grid.ColumnDefinitions>
And the code for the CollectionView is as follows:

<d:CollectionView.ItemsSource>

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</d:CollectionView.ItemsSource>
<CollectionView.ItemTemplate></CollectionView.ItemTemplate>

And the following is a screenshot of what this shows:

I want the CollectionView (which is the black area) to use the minimum width on the left. What am I doing wrong? Thanks.