As you'd expect from the descriptive references, a small block is a physically smaller motor than a big block and weighs less. It is also smaller in displacement. There is a crossover at about 400 cubic inches in the Chevy lineup, that's the largest small block and a 396 is the smallest big block. There are smaller big blocks, but they are obscure truck motors that none of us use for performance purposes.

Small block Chevys came in displacements of 265, 267, 283, 302, 305, 307, 327, 350 and 400.

Big block Chevys came in displacements of 396, 427 and 454. There are some late model 502 big blocks available also.

All these small block motors I've referenced are 1955 to 1986. There are some others that have been built since '86 that I am not familiar with, maybe someone else can jump in and help.