It's more expensive to produce something that's bigger, especially if you want the same resolution in the end product. Material expenditures would also increase per part, generally as the cube of the linear dimension. I believe Makerbot is trying to move towards desktop 3D printers, so it's unlikely they'll move to larger models. In other words, there aren't any technical reasons it can't be done. There are 3D printers that can produce complex architectural pieces the size of a gazebo (see the TED talk on the subject - very cool stuff). The problem is that such a printer costs millions, takes up tons of space, uses tons of material, and takes forever to print.