From that method's Javadoc info:
Sets the prototypeCellValue property, and then (if the new value is non-null), computes the fixedCellWidth and fixedCellHeight properties by requesting the cell renderer component for the given value (and index 0) from the cell renderer, and using that component's preferred size.
As I understand it, you've given each cell a fixed width that's shorter than the cells should be. By commenting out that line, as Rob C suggested, each cell gets its correct preferred width.