The difference between knits and wovens is fundamental to fit: woven fabrics (like poplin or denim) have little give and rely on cut for fit, while knits like jersey are formed from interlocking loops that stretch in at least one direction. That stretch is why a jersey tee feels softer and more forgiving than a crisp woven shirt.
Heavier jersey holds shape; lightweight jersey clings — which is why the same size can look very different depending on the weight.