Heavy tank
The Char 2C was one of the largest tanks ever produced. At the start of World War II, France and the Soviet Union were the only countries to have inventories of heavy tanks, such as the Char B1, T-35, and KV-1. The Matilda II was designed under the British infantry tank concept which is similar to the heavy tank having thick armor and weighing more than their other tanks. It’s usually separate because they have less firepower with their cruiser tanks (comparable to mediums) at the time having the same main armament but more machine guns. Later war examples were the German Tiger I and II, as well as the Soviet IS series. Note that "heavy" versus "medium" is more a question of tactical roles than weight; the Panther, for example, was a "medium" tank that outweighed most Allied "heavy" tanks.
Tanks in the Soviet Union
By 1935, the Red Army "... possessed more armoured vehicles, and more tank units than the rest of the world combined." But from 1937 to 1941, the Red Army's officer corps, the armour design bureaux, and leadership of the factories were gutted by Stalin's Great Purge. Approximately 54,000 officers were repressed. Military knowledge completely stagnated and armoured vehicle production dropped drastically (though still remaining the world's largest). Training and readiness dropped to very low levels. This repression continued until the eve of the war.