I think Michael Margarin has it right. While you can do the 'top down' view (i.e. if we only get 1% of the $xB market we get rich) you should do a 'bottoms up' view as well.
The importance for a startup is the ability you have to scale. Doing 1 is hard, then 10, then 100, then 1K, then 10K, then 100K etc. ... this scaling is the hardest thing to get right. A bottom's up view allows you to put milestones against the plan. At what point in the scaling do you b/e or make a profit? How will you access these opportunities? What is the cost?
While it might look like you are limiting you market, what you are actually doing is showing the practicalities of your idea.