I think it comes down to additional bending in the frame and other supports made from aluminium extrusions, there are calculators online to tell you what to expect. The increased mass of moving components which might necessitate larger motors/gearing and all that comes with that, and making a large build plate which is flat but not too heavy, and being able to heat it (probably using mains voltage heaters).
It's a classic case of size to mass ratio being a cube relationship, going to 1m from 40cm means everything is 15 times heavier!