In real estate investment the mantra is location, location, location but for a startup company it should be focus, focus, focus. Far too many companies bite off more than they can chew even before they have cut their milk teeth. While big markets numbers may turn some people on, it's always better to be a master of one.
This is what Ev Williams (founder of Blogger & Twitter) has to say about this:
“Focus on the smallest possible problem you could solve that would potentially be useful. Most companies start out trying to do too many things. Focusing on a small niche has so many advantages: With much less work, you can be the best at what you do. Small things, like a microscopic world, almost always turn out to be bigger than you think when you zoom in. You can much more easily position and market yourself when more focused. And when it comes to partnering, or being acquired, there’s less chance for conflict. This is all so logical and, yet, there’s a resistance to focusing. I think it comes from a fear of being trivial. Just remember: If you get to be #1 in your category, but your category is too small, then you can broaden your scope—and you can do so with leverage.”
Most businesses need to cross the chasm from early adopters to the mainstream and that's a lot easier when you have a whole product however small, then you can start to add +1 extensions to grow your market. The downside of trying to calve off too large a slice of market real estate too early is that you never have enough substance in any specific area to transition your product or service to the mainstream which is where you will make your money - however big or small your target market is.