I'm not certian but I think the "bigblock" term came out when chevy started making their larger series engines instead of just the 283/327/350 size.
just as a funny... I always refered to the FE ford as a FAT ENGINE
and a darn good reliable powerplant too!
they have the charicteristics of a big block in size, weight, and use, but
I just tend to think of them as a FE and it saves the confusion with the 429/460 as they are bigger in the cubic inch displacement department.