You suspect correctly. Tractors weigh more because they have steadily increased in size which decreases the number of passes a tractor needs to make over a field.
Like having a 20 inch lawnmower vs a 50 inch lawn mower. The 20 inch mower can be self propelled with a tiny HP engine. The 50 inch mowers are pretty much all riding because the extra drag and engine size increase to cut the grass is large enough that adding bigger wheels and a steering wheel makes more sense than having someone push it. Obviously, the 50 inch mower will work a lot faster.
Like having a 20 inch lawnmower vs a 50 inch lawn mower. The 20 inch mower can be self propelled with a tiny HP engine. The 50 inch mowers are pretty much all riding because the extra drag and engine size increase to cut the grass is large enough that adding bigger wheels and a steering wheel makes more sense than having someone push it. Obviously, the 50 inch mower will work a lot faster.