It all depends on the fabric itself. It is sometime possible to unshrink garments. Simply Laundry will explain in further details.
For washable fabrics like cotton, what you can do is soak the garment in white vinegar for an hour and then use hot water to wash it. If that method doesn't work, try stretching the wet garment by hand until it returns to its original size. The trick then is to hold it down with any heavy objects and allow it to dry flat.
For dry clean only garments, there is a bit more work compared to the washable fabrics. The process that should be used is called “blocking”, which helps in restoring the garment to its original shape. Basically, the process is to repeatedly steam and stretch the garment to relax the fibers.
Blocking can also be applied to your knit sweaters. All knits usually distort with both wearing and cleaning. At Simply Laundry, we would measure your garment before we start the cleaning process to ensure it is still in its original shape after cleaning.