Trees foliage in the crown (TOP of tree) can be reduced to enhance the shape of the tree, the removal of heavy limbs or limbs that are either crossing or rubbing causing definite danger to public or property. Pruning like this will not only assist the trees further development but will often encourage healthier growth, and thereby minimise potential hazards.
Trees foliage at times can over grow causing it to block light to your property. Thinning will not only open the trees canopy allowing light to pass through but will also encourage fresh growth which will better enhance the trees wellbeing and growth for the future.
Thinning will not change the overall size of the tree although it may look like that as it becomes more open. Depending on your needs and what you are wanting out of your tree will depend on how much or how little is taken.