It is easy to see why Chris' and Fred's "narratives over numbers" market sizing might be appealing. After all, as Brad Feld writes, "Almost every market sizing presentation is incorrect - by a lot. Enough to make it irrelevant." Chris writes, "you should never rely on quantitative analysis to estimate market size. Venture-style startups are bets on broad, secular trends."
This seems to make sense: tell a story because your numbers are going to be wrong anyway. But let's analyze the traditional market definitions to see why traditional quantitative methods are incorrect. Non-quantitative analysis (a story) is not enough. Market sizing has to be quantitative because it is a tool to make an investment (i.e. a quantitative) decision.