Well, O(100GB) is where I start thinking of "big data". Processing 100 GB is slow in a single "normal" machine (depending on what, sure). Between 10 and 100 I usually do it in a cluster since it is less hassle (even if I could process it locally with some tweaks or patience). Less than 10 is usually locally run unless I'm already computing something else

Not necessarily, there are other benefits to distributed processing than just parallel data processing itself. Resiliency, redundancy, much higher network throughput, etc. If you need low processing latency, as your disk size gets bigger, your bottleneck quickly becomes network throughput, not disk throughput. And if that one box fails...