Itemwidth < 30 in layout.legend?

Is there a reason itemwidth has to be at least 30 in layout.legend? I would love to be able to set the width of the legend key to 20, i.e., a bit shorter than the default 30.

From the documentation:

Parent: layout.legend
Type: number greater than or equal to 30
Default: 30
Sets the width (in px) of the legend item symbols (the part other than the title.text).