Are you using a seasoning mix or your own recipe? If a recipe, do you use any binder in it? Most mixes contain a binder of some sort, but many recipes do not. Binders such as soy protein concentrate or powdered milk, help hold juices in and result in a plumper, fuller sausage. Also, how much water are you adding? Certainly you need enough to allow your meat to flow properly, but I suppose too much could potentially evaporate away and end up in some amount of shrivelling.