Was the Panther a ‘heavy’ tank? Should it be compared to the Allied mediums or with their heavy tanks?
In terms of weight the Panther at ~45t was definitely heavier than the Sherman and the T-34 (28-33t depending on the model). The Panther was also considerably heavier than the standard German medium tanks Pz III (23t) and Pz IV (25t). So in that sense the Panther was a heavy vehicle.
However whether a tank was classified as a medium or heavy was dependent on its role in the battlefield. There are two reasons why the German classification of the Panther as a medium tank is correct:
1). Heavy tanks like the Tiger were used in specialized Heavy Tank Battalions (Schwere Panzer- Abteilung). These units were small (about 45 tanks) and were used at the points of main effort. The Panther on the other hand was used in the standard Panzer Divisions. The goal was to completely reequip the Panzer divisions with the Panther as the main vehicle but due to production shortfalls each division had one battalion equipped with the Pz IV and one with the Panther.
2). The Tiger I and King Tiger tanks were built in small numbers. Panther production on the other hand was substantial. In the period 1943-45 we have 6,132 Panthers which can be compared to 6,686 Pz IV and 1,761 Tigers.
Sources: ‘Panzertruppen’ vol2, ‘Sledgehammers: Strengths and Flaws of Tiger Tank Battalions in World War II’, ‘Waffen und Geheimwaffen des deutschen Heeres 1933 – 1945’