Talking legalities here, not "smarts."
With some minor googling California is the only place that I've come up with a displacement minimum (150cc) for riding on the Freeway (Interstate). Many states have laws against "motorized bicycles" on the Freeway which encompasses 50cc scoots. But I'm not finding a national trend mandating 150cc over 125cc. Am I missing something? What are the laws in various state?
PS-- I'm say legalities, not smarts-- I think most here would like to avoid freeways like the plague on smaller 60mph max scoots. But, as has been pointed out that isn't alway possible. As an example to get from Albuquerque to Santa Fe requires Interstate riding, or a massive re-route.