Why does a competitor rank with a new website with minimal content?
Backlinks play a big role in how well a site performs these days, regardless of content length and optimization. You can use majestic.com to view a website's backlinks, which won't provide a clear view of everything, but can give you a good idea of a website's backlink performance.
Another thing to consider is the competition level of the industry in question. Obviously, niche industries with minimal competition will be easier to rank for.
Always Consider Backlinks
In short, if a domain has many high-quality backlinks, it won't require much content to perform well. The more good backlinks a website has, the less important content length becomes.
However, content should still be high-quality if you want more backlinks. Without good shareable content that people want to link to, you're far less likely to see a page perform well.
Compare a webpage advertising a PPC or SEO audit with 300+ high-quality backlinks and only 50 words of content to another audit that has a few backlinks and 500 words of optimized content. The former is much more likely to rank high over the latter because of that reputation.
Ultimately, it's in your best interest to focus on backlinks over content, but good readable content will always be important. Practicing a combination of both will help you succeed over your competitors.