I do notice that unit is not included in the Width property and that was your problem before. Below is the Display Fields property after the configuration:
<Fields>
<Field Name="Subject" />
<Field Name="Created" Grouped="true" GroupExpanded="true" GroupSortDirection="asc" Width=" 10%" />
<Field Name="Categories" Width=" 50%" GroupExpanded="false" />
<Field Name="Modified" Width=" 100px" />
</Fields>
You would either give it "px" or "%" and can not just use a number "1" or "120". The Web Part can not guess whether it is going to be percent or pixels.