The ornithologist is a better linguist than he is an ornithologist

 By this I mean that supposing an ornithologist has an extensive vocabulary in her field: names of birds, words used in a specialized way to describe them, scientific terminology in general.  This extra vocabulary runs to thousands of phrases. But the ornithologist will never know more bird words than words in general, because the bird words are part of the vocabulary, and are added to the total.  The ornithologist also know how to form phrases and talk about other things, can follow the rules of phonology and morphology and syntax and speech prosody. He or she is an educated person, also, since learning ornithology was part of an education into many things.   

You never know more of any other category of things than you know words, since you are likely to use words for those other categories too. If the ornithologist know 5,000 birds words, they will still know 20,000 non bird words as well.   

I'm using "linguist" here in a novel sense, because I don't know what other word to use for this. 

There  could be counter examples, I guess. People not very good with their words. Usually, though, we are comparing those people to people who speak more adeptly. They are below average, but still fluent in their own language.