It is the batting that controls shrinkage of a finished quilt. If you don't want any shrinkage, use a polyester batting (polyester does not shrink) or prewash a cotton batting (you need to be sure the cotton batting is capable of being prewashed without falling apart; not all cotton battings can be prewashed).
Check the batting package or Google the manufacturer of the batting to find the one you used. Typically the manufacturer specifies that a cotton batting is expected to shrink 3 to 5%. This happens the first time you wash/dry the batting.
Assuming the quilt has moderate quilting on it (lines 3 or 4" apart), all of the shrinking in a finished quilt will be due to the batting. Whether the fabric is prewashed or not makes no difference with moderate quilting, as batting will not allow fabric to shrink more than the batting shrinks.
Fabrics washed on their own shrink at their own rate. They do not shrink in the same way once they have been quilted to batting. This is why prewashing fabric has little to nothing to do with how much a quilt shrinks.
If you pay attention to the shrinkage rate for the batting you are using, you can plan in advance how big to make the quilt top so that the finished quilt fits the bed. For example, say you want a size after washing of 90" square and the batting specifies shrinkage of 3-5%. 90" x .05 = 4.5". You will want to make the top at least 95" square to be sure it will be at least 90" square after washing. It's also a good idea to add an inch or two in each direction to account for quilting take-up; more for larger quilts, less for smaller quilts.