Being longarm quilted doesn't matter anymore than any other type of quilting, in my opinion. I have a longarm, and used to handquilt as well as freemotion on my Singer, so have done them all. I find what makes the most difference in any quilt, regarding the washing, is the fabrics themselves, the batting, and even to a lessor extent, the thread (type and tension).
80/20 will shrink about 3%. 70/30 and 100% cotton shrinks more. Poly hardly any all (some of my best looking quilts after washing are poly batting). I've read that 100% cotton fabric will continue to shrink and change when washed up to 3 times. In other words, they will continue to shrink for the first 3 washings, then after that they are considered 'stable'.
When you do wash a quilt, take it out wet and block it. I do this with any wall hangings and show quilts to help them be straight and square. It's a chore, and it's why I don't wash them often (among other reasons). Sometimes I'll dry a bit, then block. Never over-dry or the shrinkage will be worse.
I don't know if this helps you or not. I'm sorry you're surprised by this, but frankly, I'm not, as it happens most of the time. I do hope it helps some. Perhaps look for some testing results, or do some yourself, to determine what amount of shrinkage is acceptable to you, using battings that are common in your area or that you like to use. I know some 'name brand' fabrics also do continue to shrink pretty bad...
Debbie in Austin