As every farmer knows: it's not the weight that matters but the soil compaction, and that's a function of weight over surface area of the contact patches of whatever drive train your tractor has. That's also why tractor tires are so wide, and high flotation tires are commonly used.

Another way in which farmers combat soil compaction is by aeration and tilling.

It's true though that tractors are getting larger and heavier, but farmers are pretty knowledgeable about these things and usually take them into account when deciding what kind of machinery (on what kind of tires) to use for their soil, after all, if they get it wrong they may end up negatively impacting the yield of their land.

Finally, crops tend to be planted in rows for convenient mechanical processing, and while walking behind a tractor you can actually see the soil rise again after the tractor has passed, usually because the soil acts as a sponge, the tractor squeezes the water out and once it has passed the soil will spring back. It's a bit strange to realize that the ground you walk on is so springy because you normally don't notice it.