I think there is a subject. The is always used before a noun, so more is now substantive (a noun), and 'the more' functions as subject. (Cf. the poor, the haves.)That's not a sentence. I'd say fragment. We don't have a subject. If it was
"The more candy, the better".
Then it'd make sense.
As for "The more, the merrier." Even though it's a saying, it's still a fragment.