first of all, lets just talk chevy (lest somebody accuse me of not knowing the difference between a 383 big block chrysler and a sroker small block chevy).
There's a long list of "smallblock" chevy motors: 265 267 283 302 305 307 (don't get confused also an olds motor) 327 350 383 (comon stroker) and 400. These motors all look exactly the same from the outside (well, the 265 doesn't have an oil filter, minor differences). The block demensions of these motors are all identical. The bore and stroke of these motors makes the difference in CID, but they share a common "block." That's not to say you can take a 265 and bore it to a 400, castings are different, but like I said, block dimensions are the same.
Bigblock motors are a larger block, mainly for the purposes of being able to put more stroke and more bore into a given chunk of iron (without getting thin, weak areas or bores that are getting dangerously close to water jackets). Given the more spread design of the bigblock bores, the head design is much better than the standard smallblock. Bigger, better angled valves and intake runners can be used with bigblock motors, the bigger bores reduce valve shrouding. Bigblock is a more "performance" oriented design.