The reason why the M26 became a medium tank after WW2 is because of a weird, american classification system. After WW2, tanks with gun calibre under 90mm became light tanks, Tanks with a 90mm gun became medium tanks and above became heavy tanks (the 3 tank calibres of the US army became 76.2mm, 90mm and 120mm)
By WW2 standards, the pershing is however a heavy tank, because it is heavier then the american M4 sherman medium tank.