I have this problem, too. A quilting teacher told us to pin the 2 pieces together & if one is a little larger than the other, ease it to fit. Then give the area a shot of steam from the side that is too large. You'd be surprised at how this will shrink it to fit. This works if you are using 100% cotton fabric (which is what we all use anyway, isn't it?)
Try it. I think you'll be pleased with the results as long as you don't have too much to try to shrink in.