It’s pretty simple to see whether or not someone has a lot of followers. Usually people follow others through popular posts, and those people who have popular posts are either A) already famous or B) have tight mutuals (and thus easier to become famous). It’s also possible to get attention through tags, but that only works with gifs/pictures.
Why do you think the popular blogs get more traffic than any other blogs? Because they’re popular for being popular. I could find a blog just as good, or even better, than tons of blogs such as ruinedchildhood or euo (no offense to them, their blogs are awesome. They did start from the bottom like the rest of us).
It’s come to a point where a person whose popular could post the word “moist” and get thousands of notes. For that reason, tumblr is not any different than Instagram, or twitter.