Yes, unfortunately, they can shrink.
If for some reason you have to spend time on I.V. or on TPN and do not use your pouch or are on a very restrictive diet (I was on no carbs for over 20 yrs...so very low residue) then your pouch can shrink...I went down to 1/2 litre capacity...my best is over 1 litre (about a quart)...being on strait drain can do it too. You can re-stretch it by slowly increasing input or putting off intubating for 1/2-1 hr or more...but do it slowly.
Sharon