I may not be entirely right with this, but in general, the tractor concept takes into account the fact that a large number of birds in a small confined area will quickly consume and totally trash the ground beneath them. Think mud hole. So you move them around.....grazing new grass/pasture and often moving them daily. A tractor is easy to move by hand. Generally, it will not be Fort Knox. In a tractor, a flock of birds work like a plague of locusts....graze it and move on. The greens and bugs they consume are a planned for part of their daily food ration.
Then there is moveable, which is what I'm building. An secure 8' x 12' house on skids. While it can be moved around, it typically won't be more than once or twice a year. I plan to take advantage of the birds to let them work over/enhance my garden areas, then move them to new ground to do the same thing, utilizing electric fencing to confine and protect them while they are out in the open. They will trash it under controlled conditions, which I'm counting on.
If you have the room, and want to move them around with a fixed house, that is what electric poultry netting is for. It can easily be moved and expanded to almost any shape or area you want it to be, but at all times, they are free to return to the protection and shelter of their house. With acreage, you have that option up to and including your whole acre......if you want to pop for and maintain that much fencing.
A lot of small backyard coops are just that. Small and intended to confine a small backyard flock (think no more than 2 to 4 birds) with no real intent of allowing them to roam too far and still working to deal with the droppings and soiled litter that even a couple birds create.