Actually think it's better this way (the smaller size, I mean). Makes everything lighter and easier to handle, and you're still using the same amount of wood after they plane it smooth.
Yeah, that fact about the 2x4 is a classic piece of woodworking trivia. I heard it's because they plane the rough lumber down after it dries, so you start with a full 2x4 but end up with less. The actual size now is like one and a half by three and a half inches. It's one of those things that makes building anything a bit trickier if you don't know it going in.