this one illustrates it quite well:
"From bottom to top, the five layers of the Product-Market Fit Pyramid are: your target customer, your customer?s underserved needs, your value proposition, your feature set, and your user experience (UX).
In the process of trying to define and build a successful product, you form hypotheses in all five of these areas (whether you realize you are doing so or not). The Product-Market Fit Pyramid helps you be more explicit and rigorous about your hypotheses. [source]
Having said that , scale of the market plays a very important role. For e.g. one can easily start an ecommerce site (say, dogkart) and witness good number of sales in the first week itself.
But at scale, will it work ? Is the market matured enough for this to become a big business?
That is, the def of product market fit keeps changing as you scale.