It had a lot to do with the function of the tank. A Medium Tank, at this time, was meant to function in most roles, primarily in combined arms, being able to take out enemy tanks while at the same time being fast, armored, and versatile enough to perform its shock operations. In comparison, light tanks were used as infantry support or blitzkrieg purposes, and heavy tanks were primarily used for infantry support as well, but were generally too slow to lead an armored assault.

This means that Medium Tanks will have less armour, and less powerful armaments, than that of their larger Heavy Tank cousins, but will be faster. Weight is a by-product of Heavy Tanks being heavy due to all of their armor and weaponry. Usually, the weight of a tank is heavily related to its type, and in many ways, the Panther was much like a Heavy Tank than a Medium one. However, the Germans built their tanks much differently, much more skilled labor, time, and effort went into making them. They were over-engineered (at times), and very complex, which meant they were on average much heavier. To put it into perspective, you listed the two main Heavy Tanks on the side of the Allies. The German Tiger I was 54 tonnes, and the weight of the Tiger II was a whopping 68.5 tonnes. German tanks overall were much larger and much more complex, which meant they were, at the very least, much heavier than their rivals of the same class. In the case of the Panther, its weight seemed to suggest it was more of a Heavy than a Medium tank, but its function was much different.

The importance here is then that the Panther's speed was around 55km/h, and later was reduced to 46km/h in its later stages as more armor and armaments were added. The M26 Pershing had a speed of only 25mp/h, the equivalent of around 40km/h. The IS-2 was even slower at 37km/h. The Panther, therefore, was much faster than both.

The guns of the IS-2 and the Pershing were both larger than the Panther, however. While the Panther gun had superb penetration and velocity, it remained a 75mm gun, most aptly used against other Tanks. The IS-2 122mm gun and the Pershing 90mm guns meant that they had a larger explosion, and were more effective against infantry and fortifications.

The Panther is, as far as I am concerned, more aptly classified as a Main Battle Tank than as a Medium or Heavy Tank. The Panther had the armor and firepower (more or less) of a Heavy Tank but the speed and versatility of a Medium Tank. Many, rightfully, consider it a predecessor to the concept of the Main Battle Tank.