There's another factor, though, which you missed entirely: larger SSDs have muchz much faster write speeds, and also read speeds. The parallel NAND chips provide a sort of intra-device RAID
But I did mention that, for the smaller SSDs? It's also true for 250GB vs 512GB, but the main appeal of having an SSD does not get lost by getting a smaller one – latency and faster load times than with a HDD.