I'm not so sure about this part, although we still don't have enough data for a better conclusion. But theoretically these institutions bring in large amounts of money in bulk and they could actually cause bigger swings (higher volatility) by their entrance and possible panic exit in case their company goes down or is shut down by the government or things like that. This kind of "shock" to the market creates more volatility not less specially since the regulations around the world are not yet solidified.