Proportion is always important. Baggy clothes still knock as long as it's not all baggy everything. RiRi knows what works for her body type, and she plays with it. Not everything is form-fitting 24/7, but by pairing a loose tee with tight pants, or rocking an oversized men's shirt as a dress with brogues, she shows that she knows the rules of dressing well already, so she obviously gets a pass in breaking them.