I think what you are missing is that the massive whale prefers to look like the small minter, so they split their stake into 10-100 ppc fragments to maximize number of blocks per unit time. You're just looking at chance to mint one block, but there is a static block reward now (~1.26 ppc, this is a v0.9 thing) so you are rewarded both based on coinage consumed and based on sheer numbers of blocks found. If the big minter has all their coins in one output, they will only mint once every 30 days, missing out on maybe a dozen peercoin a day compared to if they split their outputs so they can mint many times a day. The risk isn't a single large output minting every 30 days. The risk is if an attacker split up a reasonable quantity into many chunks and waited 3 years, they could have the coinage and outputs to dominate the network and commit a double spend. The 90 day limit on coinage ensures that those 3 year old coins have no more chance of getting a 6 conf double spend as any other 90 day old outputs of similar size. As devs and network engineers, we then look to maximize the number of these chunks being minted by noncolluding partners in order to improve our decentralization and, consequently, the PoS difficulty.