What about Araragi's weight?
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).