True, the U.S. does conduct more worldwide activity. I suppose "being involved in world affairs" does inherently somewhat increase the odds of starting a massive war. But what in particular do you see as being especially risky of that specific consequence? Even if the U.S. is in "little wars", what about them do you see as more likely to cause a "big war" as compared to Russia annexing part of the Ukraine, assassinating dissidents in other countries, trying to destabilize regional adversaries, and allegedly things like the NotPetya cyberattack that reportedly did 80% of its damage in the Ukraine but also had worldwide effects?
It's risky because you make a lot of enemies when you're such an overreaching power. Their economy is also many times larger than Russia so the destabilization of the economy will have much more of a ripple effect than Russia's economy would. Yes, I'm counting Syria, obviously Russia bombs too but not to the extent that the US does.