It's only the controlling bit, really.

For stopping, trailers have brakes. I have an old Audi wagon, it can legally tow 750 KG with no brakes or 1900 KG if the trailer has its own brakes (750 is the absolute limit for brakeless trailers in Estonia regardless of what you're towing with, 1900 is what is considered safe with brakes for this car). Car itself weighs 1955 empty. But realistically, it could probably easily move and even brake 5 tons or more. It's a remapped 3.0 TDI, it has more torque than some brodozers with their V8s (the petrol ones, anyway). And I keep my brake system in order, even the fluid is brand new. I don't skimp on tires either. So between the trailer brakes and my own brakes, several tons could be stopped fairly quickly.

The issue is sideways forces. If I'm towing a really big trailer and there's a strong gust of wind, it'll affect my car much more than it would affect a heavy truck. Similarly in the winter if the trailer starts skidding, my car just wouldn't have the mass to control it. The engine and brakes do a great job of controlling forwards and backwards forces, but there's really no mechanism for a car to do much about a very heavy trailer enacting sideways force on the rear end of the car. That's why the trailer limit is almost always smaller than the car's own weight (tractors with semi trailers being an exception, but they're limited to 90 km/h electronically and require much more extensive training)