That's the AN-124, we see it often in Denver. It picks up rocket boosters here from Lockheed-Martin and ferries them to Florida. I can tell when one flies over... sound is somewhat unique.
It's big, but It is dwarfed by the Antonov AN-225 with 6 engines. It's the largest airplane in the world. It was designed to carry the USSR space shuttle. I think there are only a couple in existence. See below.
https://www.youtube.com/watch?v=v8d5x6uOKwA