Scale refers to the ratio of measurements of model to real when making a miniature. A scale of 1:9, as in Breyer Traditional scale model horses like this discontinued mahogany bay original finish Proud Arabian Stallion, is the largest among the model horse scales. At 1:9 scale, one measurement (of any type) on the model, corresponds to 9 measurements on the full size animal. One inch on the model would equal 9 inches on the animal. It doesn't matter what you use as props in your photo show set ups; as long as the items are in the correct proportion to the horse, it sets the scale.
Are the background bushes and plants in this photo in the right scale or proportion to the model?
Yes. Even though the "tree" behind the horse is a clump of grass, it gives the appearance of a tree in the proper proportion to the model.