Hello,
That's the default behaviour of MS Word. Although you can apply fixed widths to cells, if a string is very long without spaces, then the cell width will be increased automatically.
The addTable method includes the fitText option that can be applied to cells:
fitText bool If true fits the text to the size of the cell.
But this option is not available when transforming HTML. We have added a task to add support in the next release of phpdocx (there's no release date).
Regards.