Which valve is bigger on Pontiac cylinder heads?

The intake valve is larger. This is true on virtually all internal combustion engines. The reason for this is that it is far easier to get the spent exhaust gasses out of the cylinder since they are still under high pressure due to expansion from burning (80+psi), than it is to get the fresh mixture into the cylinder using only atmospheric pressure (14.7psi). If you are referring to traditional Pontiac V8s the intake valve was usually 2.11" the exhaust was 1.77" or 1.66" on later "smog" heads.