Data which doesn’t fulfill all of the above three characteristics isn’t considered big data. For instance the data which is not huge, being generated at a pace which can be handled by traditional DBMS and is well structured and relational isn’t considered big data. Data which fulfills one of the three Vs is Big data.
Though, anyone with specialized software and hardware products can collect big data but primarily big data is collected by researchers for research purposes and business analyst for extracting useful information from the data which leverages them to take business decisions.
Numerically speaking, data which consist of petabytes(1024 terabytes) or more is considered as big data. However, different analysts have different definitions regarding the magnitude of big data. It is also said that data which cannot be processed by a single machine, or if the data processing requires specialized tools for storing and processing, the data is called big data. Also, if you need to hire a team of data-scientists just for manipulating data, consider your data, Big data.