To a certain extent it depends on the elasticity of the fabric. If the fabric gives only a little, the jacket will need to fit pretty loose. If the fabric has plenty of play, it can be cut tighter to the body. If you live in a place where the winters get brutally cold you’ll probably want a jacket that’s loose enough to accommodate a mid-layer beneath it but not so flowy that there are too many air pockets and you end up cold.