JG,
It usually depends upon the organ and what is causing the enlargement.
In the heart and aortic artery enlargement, unfortunately, once these tissue have been stretched out, they usually do not go back to their regular size. And, if the cause of the enlargement is not corrected, then, of course, the pressure causing them to enlarge in the first place is still there.
The enlargement of the brain, again, depends upon the cause. Here, if the cause can be stopped and the enlargement is mainly swelling, then the brain can decrease in size. However, if the enlargement is due to the deposition of substances within the brain tissue, then the enlargement is usually permanent.
But, these are questions that you should be asking your physicians. They can better tell you the answers, as they know the causes and underlying conditions.
Good luck.