Displacement, bore, and stroke have nothing to do with it. Neither do the heads, or other bolt on parts. It is strictly the physical size of the bare block. Most manufacturers have 2 (sometimes more) sizes of V8 block castings, one smaller than the other. This is done so that the cooling jackets around the cylinders are never too far or too close, which would both lead to poor cooling.
For example, as mentioned, a Ford 352 is a big block, whereas a 400C is a small block. Set the two bare blocks next to each other, and you'll see that the 352 is physically bigger.
Take care,
~Chris
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