This is just my opinion but I feel RUT is less volatile than SPX because of all the tech stocks that are in the SPX so any news on the tech stock will move the SPX a lot
Another factor is the size or the underlying If you look at the RUT 10 years ago it was less than 1000 now it's almost double while SPX is more than double so a certain percentage move in RUT is going to move the SPX a lot more so you not only have to make the tent wider on a BWB in SPX but also have to keep a closer eye on it and be ready to make aggressive adjustments and maybe even reverse it the next day