it's not about putting more content in the sub directory or sub domain, because putting more content is not the key to achieve rank. We're talking about sub domain or sub directory, then I'd recommend using sub domain if you want to be recognize individually on Google serp.

As Google sees sub domain as a separate part of your website and counts its pages individually on the serp, and ranks the pages of that sub domain separately other than what pages link to your main website. Sub domain is used within your site when you think that the content nature is different from the main site content.

You can see examples of Google sites on different content, such as news.google.com, maps.google.com, analytics.blogspot.com, adsense.google.com and so on.

On the other hand, you should use sub directory if you think that it is the content similar to your main site content. And, the benefits of using sub directory is that link juice is distributed uniformly across the site. As Both have pros and cons that you should read. Read it what Mattcutts said - https://www.mattcutts.com/blog/subdomai … rectories/