My .02?
It's an old adage. Used back in the day when there were displacement limitations because of block architecture.
Big blocks were just big assed chunks of cast iron that could displace whatever their external dimensional limitations allowed based on what internal bore/stroke combination was applied. I.e. a 352 became a 360, became a 390, became a 427, became a 428. (and if you were crafty back in the day, a 428 crank used w/ a 427 bore, became a 454.) Regardless of the displacement though, every one of the c.i.'s listed above all lived w/i the confines of the same block architecture.
Just think of it as a big box built extra large so as to accomodate multiple purpose design intentions.
It was really always a Chevrolet coined phrase anyway, when the 396 showed up in the middle 60's, they needed a new catchy way to differentiate the standard boring little turd from the new bigger one.
Modern strokers have dispelled any rule of it being a size thing based on displacement, as it is now possible to get up to 490+ inches in a SBC, and what? 460 out of a aftermarket Windsor SMALL block?