It's a well known best practice to use a mobile-first strategy. What does that mean? It usually means that every feature/website/page that you build should first of all look and behave good on a mobile device and only then should it be adapted to a desktop screen.
Sure, if your ideal customer spends lots of time on their mobile phone, it's a no brainer to serve your product in a small screen format..
That might not alwaus be the best path to go down.
Consider administration interfaces for complex systems. While a simplified version can be used from a mobile device, it's usually better to have the extensive feature-set in desktop/wide-screen format.
What if you're offering a complex product/service that you cannot showcase on a small screen, then just building a desktop website is definitely a better solution, considering time and cost constraints.
And this isn't a size thing, you might have a huge screen phone that can display complex interfaces, but think about the context in which a phone is used. Think of what you want your visitors to be doing when they reach your site/product. Chances are they'll be much more focused when getting to you via a laptop browser than a mobile one.
There's a three approaches you can adopt:
Actionable advice: always think of the context from which you want your users/visitors to come in contact with your product/service; if you need them to be focused, then a desktop-first strategy might be better.