Thank you, everyone, for the suggestions. I think that I should clarify a few things, though:
1) It's a first person story, so the "little guy" and the "big guy" are both "I" in the narrative.
2) The "little guy" is actually an imitation of an actual person that the protagonist made a deal with. So during the once a month when he turns big the other little guy comes to replace him, so the protagonist interacts with a character that looks, acts, and has the same name as the guy he had been earlier in the book.
3) My "foreshadowing" was already pretty heavy-handed, I thought. I mean, I already had a scene where the little guy is fighting to keep from changing, where his limbs keep trying to enlarge and he is fighting with all he has to stay small. Then, the next day he goes to a mark on the wall and determines that despite his best efforts he had already grown some. That was why, before other people read it, I thought that it would be fine to have the character wake up in his big body the next day.
But I think that I will try a few things from your suggestions. The jewelry piece idea especially fits, since I've already got a medallion that the protagonist wears and I can just reference that right off the bat when he wakes up. I also think that I could be a bit clearer with my descriptions immediately pre- and post- change so that the change transition is really the only thing that the reader has to wrestle with instead of having a few other loose ends the way that it is currently written.
Thanks again, and if you have any other advice by all means keep it coming.