Darkside is the largest of all Pureblood (naturally occuring) Heartless. Twilight Thorn is the largest naturally occuring monster Nobody (assuming that the other monster Nobodies, who's abilities mirror those of the Organization XIII members who control them (ex. Xigbar and Sniper Nobodies), were actually created or in some way formed by the Org.). They are not Sora's dark side or Roxas's dark side or each other's dark sides or any other combination you can think up. Roxas wasn't Sora's dark side either, though he WAS Sora's Nobody.
By the way, Namine'-Kairi, I like don't mean to like totally flame and I like know that you're only like 13 but like if you haven't yet then you're like totally going to learn that talking like this is like really annoying and people are going to say that this is like really infuriating and other people are going to be like, "Dude........it like so IS!" And again, I totally do not mean to be all like "STFU" and everything because I'm like not saying that but if you're like actually reading this then you like really gotta agree that talking like this is like so annoying.