F-650 and F-750 have always been in the Medium Duty Truck category.
Ok, Wikipedia shows the 750 as being Heavy.
https://en.wikipedia.org/wiki/Truck_classification
US truck class Duty classification Weight limit [1][8] Examples
Class 1 Light truck 0–6,000 pounds (0–2,722 kg) Dodge Dakota, Chevrolet Colorado/GMC Canyon, Toyota Tacoma, Nissan Frontier, Ford Ranger[9]
Class 2 Light truck 6,001–10,000 pounds (2,722–4,536 kg) Dodge Ram 1500, Chevrolet Silverado 1500, Ford F-150
Class 3 Light truck 10,001–14,000 pounds (4,536–6,350 kg) Dodge Ram 3500, GMC Sierra 3500, Ford E-350, Ford F-350, Hummer H1
Class 4 Medium truck 14,001–16,000 pounds (6,351–7,257 kg) Dodge Ram 4500, GMC 4500, Ford E-450, Ford F-450 [9]
Class 5 Medium truck 16,001–19,500 pounds (7,258–8,845 kg) Dodge Ram 5500, GMC 5500, Ford F-550, International TerraStar [10]
Class 6 Medium truck 19,501–26,000 pounds (8,846–11,793 kg) Chevrolet Kodiak/GMC TopKick C6500, Ford F-650, International Durastar [11]
Class 7 Heavy truck 26,001–33,000 pounds (11,794–14,969 kg) GMC C7500, Ford F-750 [12]
Class 8 Heavy truck 33,001 pounds (14,969 kg)+ International WorkStar, Kenworth T600, Kenworth T660, Kenworth T680 - Semi-trailer trucks fall into this category
Class 9 Super-heavy / special duty truck 33,001 pounds (14,969 kg)+ Usually class 8 truck with special duty characteristics, e.g. - International WorkStar, Western Star 6900 (6900XD or 6900TS).[13][14][15][16][1