There's a concept known as the "armor-gun race"; its a good way to describe how armor is in a constant race to make enemy weaponry obsolete, and how guns are in a constant race to make enemy armor obsolete.

The constant shift upwards of both meant that what was a heavy tank in 1945 may be considered a medium in 1950. With the M26 in particular, its mobility and power-to-weight was more akin to a medium tank to begin with. The 90mm cannon, despite its high velocity was comparable in caliber size to emerging cannons equipping other tanks of the period; the French 105mm, the T-55's 100mm, and the lower velocity 85mm that equipped the T-34/85; the Pershing's main victim in the Korean conflict. Conceptually, the M26 was meant to replace/improve upon the M4 and thus take over its missions; a range of mission made for a Medium tank's agility and power-to-weight ratio.

I believe as far as Korea the M26 was still being referred to as a Heavy or Medium interchangeably, but as the US entered the 60s, new tank designs and concepts as well as the rapid obsolescence of the M26 lowered its classification decisively.