This option is controlled through the Format (Caption) menu.
Captions are rows without data. Normally the value set label is used as a caption before the stub group and the caption text box is restricted to the same size as the other stub text. However the caption text box can be widened to the width of the table by using its Span fields option. Right-click on the caption, choose Format (Caption) to bring up the Format (Table Part) dialog and check "Span fields".
An example:

Select the "Span fields" option from the Format (Caption) menu.

The result:

Note: The horizontal alignment of the caption was also changed to "center" to demonstrate the size of the associated text box.
By default, captions do not span fields. You can make all captions in all tables in the file span fields using Preferences and Default Formats.