Actually a cotton shirt does not shrink do to washing as much as it has to do with the drying process. Many people (myself included) lead a busy life, therefore taking the clothes out of the washing machine and throwing them in the dryer, without paying attention to the setting of the dryer. If you are drying a newer shirt and you dont want the shirt to shrink as much, try using a lower heat setting on your dryer, also DO NOT over dry your shirt, as this will cause the shirt to shrink more do to the evaporation of natural mostorizing proporties of the cotton itself. Also previous to buying a new garmet check the tag, now a days a lot of clothing is coming (pre-shrunk) that will save the guess work if you're between sizes. As for wearing a shirt that is too small for you, absoulutely will it expand. Your body temperature is 98.6 degree, the average person will wear their shirt for atleaste 8hrs a day. So combinding the longevity of use and the body exposed to the shirt, it will have the opposite effect as "over drying" your shirt.