Afaik, the ruby tiara must be personal property of the King, as he bought it from his cousin count Michael Bernadotte. We never heard that the tiara was added to the Bernadotte Foundation. Anyways, making a second, wider frame for the tiara wouldn't change the jewel at all. Additional piece could be added to the sides without permanently attaching them to the jewel, so the historical design wouldn't be touched.
Also, Countess Marianne has worn the tiara in a different way, lower around the head. It looks like the frame was more open back then. Once she even wore it tilted back with the tiara curving around the head like a headband. So the frame must be either flexible, or there is a second frame to it. And that suggests, that Queen Silvia prefers the rather narrow diameter of the tiara nestled on the top of her head.