> I tell you about seeing a rare and elusive jabberwocky, and I told you it was deceptively [small | large]. Without further elaboration or context, you may struggle to deduce whether I mean that the jabberwocky is smaller or larger than expected or than it appeared, or both, or simply that it was smaller or larger than expected for jabberwockies, given just that single sentence.

I had no idea this was even ambiguous before your comment.

I've hitherto only considered the possibility that "deceptively large" means "large, but appeared otherwise. Having now googled this, it seems there's some debate about it, and while I can see both arguments, I think this is down to incorrect usage muddying the water.

In "A is deceptively B", "deceptively" is an adverb, so adding extra definition to the manner in which "A is B". Short of ignoring grammar rules entirely, I can't see a good case for arguing the opposite, even though it seems that examples exist where the sentence only makes logical sense by interpreting it as a negative.