I agree with the PP. The X is supposed to be for kids who are a little heavier set.
If you're looking at jeans, you might try getting the adjustable waist jeans. DS is very trim/slender, but too large for "slim" sizes. I was able to get a full year out of his size 4 jeans that were adjustable waists. I started by rolling up the legs and tightening the waist to the tightest setting last fall. By the end of this summer, he was on the loosest setting and had the legs unrolled.