There can be an assumption that knitting your ribbing with a smaller needle automatically makes it tight. It doesn't. It just allows the ribbing to lay nice and flat, and gives the sweater a more professional look. Using the same size needle as the body, and/or trying to make the ribbing looser can end up causing the ribbing to roll or look sloppy. Another thing to keep in mind with most sweaters is that - as long as the sweater itself fits you - the ribbing is only likely to be tight if you bind off too tightly. It's all about that bind-off. I don't recommend a stretchy bind-off, but a medium/loose regular bind-off is usually plenty. If the ribbing feels restrictive, then it's likely that it was bound-off too tight, so I recommend trying it again. Blocking is a great way to coax the ribbing where you want it, even if you used a smaller needle to work it. Just make sure to soak the sweater for at least 30 minutes to let the fiber absorb the water thoroughly and when you pin it flat, be sure to pin the ribbing so it's in alignment with the rest of the body. CAN you use the same size needle as the body? Yes. But it's almost never necessary. If the ribbing is tight, always look to the bind-off as the culprit.