I think the large rock is ok, I think that weed control and borders to contain the rocks might be the easiest fix. Borders will contain the rocks as it now 'flows' out into the road. Also consider removing all the small plantings, or at least concentrate them in one area for effect, maybe in the round area shown in the first picture behind the last car in the pic.

You could use all the larger rocks in your mix to create the border and then add more gravel, but I would not add gravel in with the medium and smaller rocks. Also remove weeds and if you are laying down new gravel, put down landscape cloth to prevent weed growth. In the least, I would remove all the current rock, lay down landscape cloth to prevent weeds, lay the largest rocks back as a border and then place the smaller and medium rocks back into that area, and move all plantings to the circular area as shown in the first pic.