These are all adjustable, similar to APR %, witnesses can adjust block size or RC costs as far as I know tho I'm happy if someone reading this may correct me. I think RC's even have a flexibility of their own where they adjust based on recent demand, I know that when claiming account credits if I were to use a big account, say @ocdb to claim hundreds in one go and before I do that @acidyo was saying I could claim 40 accounts, it would definitely affect it to after the ocdb claims show I can only claim 20-30 accounts at this time. Same thing goes for RC for comments, votes, custom json's I believe.