There are alot of different grades and qualites of muslin. Some of the very inexpensive muslin I have seen shrink ALOT. Even the preshrunk - perm press muslin still has a bit of shrinkage. So the answer can vary widely as to what to expect when you wash. As a general rule I buy the same manufacture and code when I buy , so I can have predicatable results. Do note that a manufacture can have several different grades/quality of muslin so do note the info on the end of the bolt when you buy.