The biggest cargo military aircraft in terms of MTOW is the Antonov An-225 Mriya (NATO reporting name: Cossack), which is a strategic airlift cargo aircraft developed by Antonov Design Bureau in Ukraine. The An-225 is the heaviest and longest aircraft ever built, with a MTOW of 640 tonnes (1.41 million pounds) and a length of 84 meters (276 feet).

The An-225 can carry a payload of 250 tonnes (550,000 pounds) and has a cargo compartment that is 43.35 meters (142 feet) long, 6.4 meters (21 feet) wide, and 4.4 meters (14 feet) high. The An-225 is powered by six ZMKB Progress D-18 turbofan engines and has a range of 4,000 kilometers (2,500 miles) with its maximum payload.

The An-225 was originally designed to transport the Buran space shuttle and other components of the Soviet space program. It made its maiden flight in December 1988 and entered service in January 1989. However, after the collapse of the Soviet Union in 1991, the An-225 was put in storage until 2001, when it was refurbished and converted for commercial use by Antonov Airlines. Since then, the An-225 has been used to transport various oversized and heavy cargoes around the world, such as generators, turbines, locomotives, wind turbines, military vehicles, or humanitarian aid.

The An-225 is currently the only operational aircraft of its kind in the world. There was a second An-225 under construction in Ukraine, but it was never completed due to lack of funding and demand. However, in 2016, Antonov signed an agreement with China’s Aerospace Industry Corporation of China (AICC) to resume the production of the An-225 in China. The status and progress of this project are unclear as of now.

So the Antonov An-225 Mriya is the biggest cargo military aircraft in terms of MTOW. But, it is not an active military aircraft but a commercial one operated by Antonov Airlines. The biggest active military cargo aircraft in terms of MTOW is the Lockheed C-5M Super Galaxy (NATO reporting name: Galaxy), which is a strategic airlift cargo aircraft used by the United States Air Force (USAF). The C-5M has a MTOW of 381 tonnes (840,000 pounds) and can carry a payload of 129 tonnes (285,000 pounds).