Originally was going to be a comment on @Euzones' answer, but it may fit better as an answer of its own.
In Nisemonogatari episode 9, Araragi states that he weighs 56kg. The extra kilogram could be some kind of residual effect, entirely natural, or a somewhat silly lie.
Although he attempted to back out of riding on Karen's shoulders on the pretense of having gained weight from "studying too much", he was quite enthusiastic about this only ten seconds prior, with his second thoughts coming after she squatted down in order to pick him up. In light of the director's choices of camera angle, the events of the previous episode, and her later complaint of such, it seems likely that his hesitance is instead because he has suddenly gained an erection which he would prefer not to press against the back of her head. (Perhaps the light novel could clarify this?)
When he does run out of excuses, he awkwardly proceeds to willingly mount her, which would contradict his ongoing concern with protecting his sisters from knowledge of aberrations if he were exceptionally heavy.
In conclusion, at that point he no longer bore most if any of the extra weight.