I'd suggest to read this:
Big data - Wikipedia, the free encyclopedia[
^]
Wiki wrote:
What is considered "big data" varies depending on the capabilities of the users and their tools, and expanding capabilities make big data a moving target. "For some organizations, facing hundreds of gigabytes of data for the first time may trigger a need to reconsider data management options. For others, it may take tens or hundreds of terabytes before data size becomes a significant consideration."
As you can see, there's no chance to define the portion of data which is identified as "Big data", because it depends on:
1) who is judging (person, organization/concern)
2) what technology is used for storage data (text file (txt, xml, csv), media files (such as mp3, video), etc.)
3) what devices is used (mobile device, local computer),
4) what technology is used to connect devices (Lan/Wan/WiFi)
5) etc.
My personal scale you can find in the comment to the question. Do not tie yourself to this scale, because
as many people you'll ask as many answers you'll get.