Original sentence: German Panzer IV tanks were often referred to as 'heavy' tanks in 1939 and 1940, because they had high-calibre armament designed to attack infantry positions.Soviet IS-2 tanks were classified as heavy due to the strong armour and the large 122mm gun, although they weighed only about half as much as a Tiger and about the same as the German Panther, classified as a medium.Soviet T-34-85 medium tanks used 85mm guns, comparable in calibre to 88mm guns on Tiger heavy tanks
Edited sentence: German Panzer IV tanks were often referred to as 'heavy' tanks in 1939 and 1940, because they had large-calibre armament (for the period), designed to attack infantry positions.Soviet IS-2 tanks were classified as heavy due to their strong armour and large 122 mm gun -  although they weighed significantly less than a Tiger and about the same as a Panther, which was classified as medium.Soviet T-34-85 medium tanks used 85 mm guns, comparable in calibre to 88 mm guns on Tiger heavy tanks