In the document (
http://scholar.lib.vt.edu/theses/available/etd-07152001-172453/unrestricted/chap...), page 2 second last paragraph it says,
"This makes the design of the large signal class-A amplifier possible
using the small-signal S-parameters. However, for class-AB, B, or C, the small-signal Sparameters are not suitable for design purposes".
Hence the moral of story:
1)Class A => Highly Linear => small s parameter = large s parameter
2)Class B,AB,C => Non linear => small s parameter =/= large s parameter
3)Large signal s parameter computation depends on input power as well as bias
4) Small signal s parameter almost fully dependent on bias