One thing that gets mentioned here is the strike zone expanded in 1963
How much did this help Koufax? What about the entire league?
Here is the change in some stats going from 1962-63
Hits per 9 IP) 6.5-6.2
HR per 9 IP) 0.6-0.5
BB per 9 IP) 2.8-1.7
SO per 9 IP) 10.5-8.9
Hits per 9 IP) 9.0-8.3
HR per 9 IP) 0.9-0.8
BB per 9 IP) 3.3-2.8
SO per 9 IP) 5.6-5.9
So he improved in walks alot more than the league average, although he had been improving for some time. Starting with 1958 he had 6.0-5.4-5.1-3.4-2.8. But the drop in 1963 is still very big and it is hard to keep lowering that (unless, perhaps, there is some other big change, like the strike zone).
But he went down quite a bit in SO per 9 IP while the league went up. On HRs, he had about the same improvement as the league. Why would he see such a big improvement in BB rate but a big decline in SO rate?
Koufax fell in BAbip from .268-.241, more than the league, .287-.275
If we do % of batters faced (removing IBBs), we get
The league improved more on SOs and HRs and Koufax improved more on BBs.