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.
Regarding the SSD, for Windows a 120GB or even a 60GB SSD would be enough (but those are also slower). 250GB is already a lot of additional space for games – again, it depends on the usage. A couple of games is what is the usual way of thinking for what a SSD should be used, and for that 250GB is just right. But if one wants multiple modern AAA games on it at the same time 480GB sure is more useful.