My first question is do you put weed killer on your lawn? If you do, it can stunt the growth of your trees. The roots of your trees reach out to the dripline (where the tips of the branches end), and they take in anything you put on the grass. If you regularly use weed killer of any kind around a magnolia, it will not grow. Pre-emergent is the worst. If that is the case, there is nothing you can do but stop using the weed killer and letting the tree grow out of it. Pre-emergent weed killers have growth retardants in them, and while they won't harm established trees, they can keep sensitive trees like magnolias from thriving. I'm sure someone will come along and tell me that is chemically untrue, but practicing horticulture in the south for over 40 years, I learned a few things about magnolias. You can't believe everything chemists say about their products. Now, to try to fix it. It can't grow if it can't get nutrients. Sprinkle a cup of Epsom salts around the tree from the trunk out to the dripline. Epsom salts helps plants make feeder roots. It may be that your tree's roots have hit hardpan and can't get any further down. This is very common in FL, where lots are built up with fill dirt, which is mostly marl, with only a few inches of topsoil on top. The Epsom salts will help it make little roots that can gather nutrients. Wait until after it is done blooming, then go out to the dripline, then add 6 inches (so 6 inches out from where the branches end) and dig a trench outward 12 inches wide and 8 inches deep. Fill the trench with compost, leaves, any kind of organic matter you can get. If you can afford Black Kow, that is perfect. DO NOT add fertilizer. This gives the roots some really good space to grow into. After it blooms this year, start feeding it high-nitrogen fertilizer monthly until October, like a lawn fertilizer, just a handful or two. Spread it from 6 inches out from the trunk to the dripline, and water in well. This will spur it to put out vegetative growth. If this doesn't work, you can try root pruning it in the fall, but this usually works. I learned this from a very wise man who worked with plants for over 40 years. I've seen this technique work wonders.

I think you have to consider some important factors in determining how much the tree expands horizontally. One is the particular culitivar. I see big differences here, some cultivars will grow branches literally pressed against the ground, and others are much more vertical. Other important factors are climate (heat zone, rainfall, etc.) and soil conditions. Here, I would classify most grandifloras as 'moderate' growers, not slow. I had one cultivar, which I would classify as FAST, but unfortunately, it did not cope well with...heavy, wet snow, then Hurricane Sandy. (It was a 'Green Giant', nice, but they have almost no brown fuzz on the leaf underside, which I think is an attractive characteristic.) The one is the picture is the cultivar 'Victoria' (developed in B.C., Canada). Leaf buds are reddish (as on Ficus elastica). It and my 'Edith Bogues' will put out good horizontal growth. My Teddy Bears are very vertical.

As long as it is done properly. Most trees in a landscape setting are planted before they have developed any permanent branches (branches that you won't ever remove). The temporary branches (branches that will be completely removed while the tree is young) should be subordinated (cut back but not removed) or left as is, until the trunk develops a good taper. I usually leave temporary branches on until they are about an inch in diameter, so I don't have very large wounds to heal. young tree pruning

Mine was planted from a 3 foot b&b in 1987. It is ca 42" in circumference @ 2.5', below where it begins to branch. About 13.3" dia. It's at least 25' tall, it lost 5' or so a year ago Feb in an ice storm. I'm in MidTenn away from any heat islands. When planted zone 6b, now 7a, according to new zone map, which lies lol.