Yarn Undyed superwash aran?

I've just recieved a big parcel of yarn from Yarn Undyed, and got some superwash aran for making some things for SIL's baby. But the yarn is really quite thin, certainly not what I'd call aran weight - I have DK yarn that is thicker than it I guess I'll have to see how it knits up, but has anyone else had the same problem?

I haven't had any probs from Yarn Undyed but I've never ordered any superwash from them.
Have you done the wpi thing to figure out what weight the yarn is?
Maybe Yarn Undyded packed the wrong yarn and if you were pretty sure the weight is wrong you could give them a ring?