The name, big five, is in some term coined with origins from professional hunting and not necessarily the size of these five animals.
There are also a lot of other interesting and exciting animals but the members of the big five were chosen for the difficulty and risk in hunting them, especially when stalking them on foot, and the degree of potential danger involved being super high. The big five are also some of the most popular species to hunt in Africa.
During some of my game viewing safaris I have noticed how most tourists marvel, getting really interested and evoked in seeing the big five. It is also enjoyable and exciting when we engage in-depth discussion with guests helping to broaden their insights on the habits and habitat of these animals.
Some safari tour operators have adopted the use of the the term "big five" as a marketing term and promotional purposes of tours. This has for sure, made the concept big five so well known among safari travelers.