So... I've long been puzzled by this and thought I may have reported it and gotten a good reason why it is the way it is. So instead of submitting it as a bug, I'll just ask:
When I make a button a three-button button, naturally the LMB child button takes on the label of the former single/standalone button. When I add a second RMB button to it, and give it a longer label than the parent three-button, the parent three-button button on the toolbar expands in width to match the label of the child button. This is ugly on the toolbar and just doesn't make any sense. I hoped that the 'Minimize menu width' option when editing the parent button would do something useful here - but no.
So - is there a good reason for this behavior I'm ignoring, or could this be considered a bug and/or otherwise something to be changed? I don't see any good reason why any child item under a three-button button should influence the width of the parent button.