As someone currently working my way through OMSCS, I imagine that the incremental cost of adding students is very low. I don't know exactly what ratio GT uses, but the only non-digital scaling they do is TAs. With more polishing of the online content and improvement of the auto-graders, they could probably push the ratio down pretty low. Just have to figure out how to do it without compromising the perceived quality of the education. And TAs aren't expensive in any case, I'm pretty sure they are compensated pretty poorly.