Small-wheeled bikes actually have certain advantages.

They also tend to be more maneuverable and nimble, better for navigating tight and twisty trails. Also easier to jump and do tricks on. And, of course, all else being equal, they'll be lighter than a bike with larger wheels -- because, you know, wheels have mass and bigger wheels have more mass.

Of course, there are real advantages to bigger wheels as well. Smoother ride, faster when coasting downhill, better able to handle bumps, better able to handle soft and loose surfaces, wider contact patch for more traction, etc. So choosing the right wheel size for you comes down to personal preference and where you'll be riding.