I think those classifications are somewhat archaic nowadays. AFV's are classified by their battlefield role more than strictly their weight. For example main battle tank, infantry fighting vehicle, armored personnel carrier, etc. In order to go toe-to-toe with other armor, tanks need to be heavy and I'm not sure there's any modern ones that fall into the "medium" category. However, if you compare the ultra-modern ones to mid-cold war ones (M1 to T-55, for example), they can certainly be labelled "heavy" and "medium".
WWII tanks are much easier to classify according to weight: M4, T-34, Pz IV, Cromwell are all "medium"; decent armor and mobility, reasonably good gun. Heavies were for breaking through (KV-1, JS-2, Tiger I/II, Churchill) lines, where thick armor was needed more than speed. Light tanks were scouts and breakthrough exploitation vehicles(Stuart, Pz II, etc.). I think the only thing that fell into the "super heavy" category were the silly German monstrosities like the Maus and E-300(?) and Jagdtiger, the last being the only one to see real use IIRC.
That's how I understand it - I'm sure some real tread-head can shoot all sorts of holes in those distinctions of course.
_________________ Let us playtest like the Greeks of old... You know the ones I mean
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