Sometimes "spreading" means that the shrub suckers and spreads that way. Other times it simply means that it gets wider than tall.

And what does your title have to do with your question?

I think yaupon will be easier to train to a specific shape than boxwood.

As lacey says, a spreading shrub tends to grow wider than tall. Other shrubs tend to grow taller than wide. Just depends on the shrub's own propensity.