The bushy plants in the stores got that way for several reasons. Often, there is more than one plant put into a pot to help fill out the pot. In addition, larger plants are often propagated from heavier or thicker stock cuttings and then are grown in ideal conditions that a greenhouse can provide, but a residence cannot. For example, in a greenhouse, plants get light from the top and all sides, but in a house a plant usually gets light only from one side. That makes a huge difference in the way that a plant grows.
In general, the more light a Schefflera receives, the thicker the stem will be. A thicker stem means the plant will be able to support itself. You didn't specify how much light your Schefflera receives so it is hard for me to assess, but I suspect that yours is not close enough to a window.
Most Scheffleras do not branch spontaneously. They tend to grow straight up, as yours has. Sometimes pruning a Schefflera back will produce some branching, but that is very iffy. Certainly pruning will keep a plant from becoming too all and top heavy. You can prune back as much as you want. New growth will emerge just below the point on the stem where you make the pruning cut.
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