I'll have to look into it. There's no option now. The width is determined by the font size. But the table widths aren't absolute - they will "shrink" if necessary to fit into a smaller location (meaning any whitespace in between columns will be shrunk). You could force that to get the chip amounts closer to the names.
For example, if you have:
<seatingChart format="diagram" columns="3">
You could try:
<div width="500px"><seatingChart format="diagram" columns="3"></div>
And then play with the width value to see what works.