What about Araragi's weight? At the end of Bakemonogatari episode 2, Araragi claimed his normal weight is supposed to be 55kgs, but on the weighing scale, it showed 100kgs. He then said something about the crab and the episode ended. There was no sign as to what happened to the weight difference in the entire series.

I've only watched Bakemonogatari, so is there an explanation or something in the later series? Or the anime has mistaken it or ignored the fact of Araragi overweight?
I have watched all of the Monogatari series until the one before Owarimonogatari. I can tell you that Araragi's weight difference in episode 2 was never mentioned again. Throughout the entire series, only a few of the episodes (or maybe just one episode, not sure, it's a very long series) mentioned about the crab but only very briefly, didn't even talk about the weight problem.

That said, I don't think the anime made a mistake or ignored that fact. This is just how the Monogatari series is by nature. It tends to leave some stuff open-ended in kind of a humorous way. Though I watched back that episode and in the end, it did say that the reason for his weight increase was because the God was careless (as they don't really care about human's emotions, etc).