The more, the merrier!
a sentence? It doesn’t seem to have a main verb, so I’m inclined to say no, but it certainly functions as a sentence in everyday speech.
I can think of three ways of analysing it:
- It’s a sentence with no main verb. Is this even possible?
- It’s a sentence with an implicit verb: something like The more we have, the better!
- It’s not a sentence. But then, what is it?
As I parse this, it’s option number two. Context dictates the way I fill in the ellipsis. “The more [we are], the merrier [we are]” and “The more [we have], the merrier [we are]” are two obvious possibilities.
I perform the same sort of analysis on the ellipsis in your question title: “If [it’s] not [that], what [is it]?”
The only justification I have for claiming that these are ellipses is that I can’t make sense of them in any other way.
Option number one is not possible in my dialect of English, although it may be possible in other dialects and is certainly possible in other languages. Option number three remains possible. However, option number two is so easy that I don’t see a reason to give option number three any serious consideration.