Brief Answer:
Thoughts on this
Detailed Answer:
Hello XXXXXXX
As it just happened last week, it's possible that there is still swelling and that the tonsil may still decrease in size. From your description, basically it sounds as though you did a bit of surgery on that tonsil, so it will need to recover.
It may also be possible that that tonsil had/has larger crypts, which may be why a stone formed there.
Without seeing it, I can't say for sure. You may want to consult, in person, with an ENT (ear nose throat) doctor (also know as otolaryngologist) if the size isn't diminishing.