This is simply a question of engine families. NOTHING more. As defined by the respective companies (in this case Chevy and Chrysler) they both produced small block and big block engine families. Which engine falls into which category has nothing whatsoever to do with bore size, crank throw, and certainly not valve size. It is all about the physical size and configuration (family) of the engines.
Chrysler 383's are "B" engines. The "B" engines were the 350, 361, 383, and 400's. There was a version of the 383 that was a "RB" (Raised B) in '59 & '60 but you'll likely never see one of these.
The "RB" engines were the 413, 426, and 440's. They are taller to accommodate the longer 3.75" stroke common to all RB's.
Just for reference, Chrysler's small block engine families are called "A" and LA". The "A" engines came in a plethora of CID versions and you can look them up if you're interested. These are the Polyspherical head engines that were phased out in '67. The "LA" engines are the 273, 318, 340, and 360 engines that we are all familiar with.