First of all, welcome to BYC!Currently I have 1 Californian buck that is just reaching breeding age and will probably replace my current buck, which is a 1 year old Broken NZ/Californian mix named General Lee. Although he has a great temperament and has consistently thrown kits of 8+ (last was 11) I am finding that they are a little smaller than I prefer at 8 weeks. 2 lb 10 oz. this could be due to the large number of kits per litter, but I feel that a pure Californian buck might have a better chance of giving me that extra lb or so by 8 weeks. I also have a 14 week old rex buck and doe that I may raise to fruition.
I wouldn't eat your current herd sire just yet. For years, I have heard about studies about the growth rate of litters; one study in particular stands out in my mind. The researches were weighing kits during the nursing phase of their growth, and they came to the conclusion that the doe only produces a certain amount of milk as she nurses. If she is nursing a small litter, each kit grows faster; if there are a lot of kits in the litter, each kit grows slower. The weight of the individual kits varied, but the weight of the litter as a whole was pretty much constant, regardless of how many kits were in the litter. The conclusion was that the optimum size for a litter was about 6 kits. In my own rabbitry, I have seen evidence that their conclusions may be correct. When one of my does is nursing only a couple of kits, they grow outrageously fast; there seems to be a risk of deformed leg bones when they grow that fast. I have long made it a habit to breed several does at the same time, so I can put small litters together and "farm out" a few from the larger litters.
If your buck is productive and has a good temperament, I'd definitely keep him around a while longer. If anything, it may be your does you should be looking at. Now, if the same does produce the same sized litters with another buck and they grow faster, and you can't say that there was anything else that changed (no feed changes, same weather, etc.), it could be the buck . . ..