Making a landscape larger doesn't necessarily pertain exclusively to the surface area. The brain also deduces space size based on height. By adding visual cues that draw the eye upward, you can make a yard that's limited in square footage appear larger. Trees are an excellent addition for adding height to a garden.
Take care in choosing tall trees, however. Trees' growth habits can take different shapes, such oval, round, upright or conical. Some trees will grow outward as much as upward, a characteristic you'll want to avoid with a yard you're trying to make larger. When adding height in a small yard, you'll want upright and oval plants that will sacrifice the least amount of available space. Evergreens like Leyland cypress and deciduous trees like the Eastern redbud will both grow up, though not as far out. Plant carefully and you'll have the best of both worlds -- a garden that appears larger without giving up planting space.