My Generation?

Having been told my whole life that my generation, the boomers, are the largest in history, I wonder how it's now the "millenials"?   Boomers are a somewhat defined historical event - WWII vets coming home and having families: 1946-1964.  Millenials are what?  And if they start in 1977, that leaves Gen X with only 13 years, and leaves out Gen Y and other iterations of generations I've heard over the years.