X86 Can push / pop be less than 4 bytes?

Hi, I am reading the x86 tutorial from the University of Virginia, and it claims that pushing and popping the stack either deletes or adds a 4-byte data element to the stack.

Why is this set equal to 4 bytes? Can this be changed, could you save the memory on the stack by clicking on smaller data items?

The manual can be found here if someone wants to view it: http://www.cs.virginia.edu/~evans/cs216/guides/x86.html