The fact is most everyone reads things and consumes everything on their cell phones. Which means shorter posts do well for mobile optimization.
Remember that over 90% of people out there are consuming content via a mobile device (mainly their phone). So be sure to write shorter concise blog posts and even social media posts (and yes SEO should be carried over to your social media)
A 300-500 word blog post is much easier to consume on a phone then something longer.