From The Economist:
Stephen Fry, whom I always enjoy, makes a claim (at about 6:10 of the video)
[English] certainly has the largest vocabulary … by a long, long, long long, way. Rather as China is to the rest of the world in population, English is in the population of its words.
There's a longish answer. For the summary version, skip to the end. For the really short version, though, the answer is “Sorry, Mr Fry.” English is certainly rich in vocabulary, but this claim is nearly always made by enthusiastic lovers of English who don't really know how the many varieties of language beyond English work. It's not that another language has more words. The comparison simply can't be made in any agreed apples-to-apples way.
The simplest problem is inflection. Do we count “run”, “runs” and “ran” as separate? The next problem is multiple meanings. “Run” the verb and “run” the noun: one or two? What about “run” as in the long run of a play on Broadway? Different enough from a jog around the park for its own entry? Different enough from a run in cricket?
Do we count compounds? Is “home run” one word or two? Are the names of new chemical compounds, which could virtually infinite, words? What role does mere orthographic convention play? Is “home run” two words, but “homerun” (as it's often written) one? What sense does that make?
More here.