Normally a magnolia is not going to need pruning. However to get more fullness you could lightly snip back the end of the shoots to encourage branching. Just don't cut back beyond the green foliage if you choose to do this.
A magnolia will develop its shape and fullness in part from its genetics and in part from the soil and growing conditions on the site. If you provide light applications of fertilizer periodically from March to August and supplemental watering in the hot dry summer months you should be able to get more growth from the tree.
Magnolia in not generally that well adapted to your area of the state as soils are often higher in pH and more clayey.
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