Buy it so it fits over your jacket. The reason i say that is because it canget really coldcold in some weather and you will need the extra warmth. Also if you have ever riden in the mountains you know the wind can be cold and the rain gear can act as a wind breaker. Even if your jacket is off, it won't matter if it's a little large on you. I guess what it really comes down to is that you have far more option in the times you can wear it..
Life is Great...We all Ride!!!! in all kinds of weather....
never thought about wearing over leather.Man good way to sweat off 30 lbs..I carry heavy shirt intop of bags for warmth if needed and if raining and cold I just throw it on before rainsuit.