IMHO it's not relative all and is pretty clearly defined - "big data" refers to technologies and processes for analyzing data that doesn't fit on a single machine, and only to that.
It is often misused to mean anything that doesn't fit on an A4 sheet of paper, but for many companies the largest dataset that they have fits in the RAM of an ancient laptop.
If you need to analyze a couple million sales transactions or a dozen gigabytes of web visitor logs then the most appropriate methods won't include 'big data' in their descriptions.
I agree of course (I'm in science at the moment), but my day would be a lot more annoying if I corrected every misuse of the terms "data mining", "big data" or any other technical/scientific term that has been adopted by the press.
It is often misused to mean anything that doesn't fit on an A4 sheet of paper, but for many companies the largest dataset that they have fits in the RAM of an ancient laptop.
If you need to analyze a couple million sales transactions or a dozen gigabytes of web visitor logs then the most appropriate methods won't include 'big data' in their descriptions.