Each parameter passed to a routine is guaranteed to decrement the stack pointer by a certain minimum amount. This behavior varies from one operating system to another. For example, passing a byte as a value parameter to a routine could either decrement the stack pointer by 1, 2, 4 or even 8 bytes depending on the target operating system and processor. The minimal default stack pointer decrement value is given in Appendix H.
For example, on FreeBSD, all parameters passed to a routine guarantee a minimal stack decrease of four bytes per parameter, even if the parameter actually takes less then 4 bytes to store on the stack (such as passing a byte value parameter to the stack).