That first novel is often a whopper. We pack in every idea spawned by TV, books, games. Into this brew we drop a boatload of gonzo characters who all want equal time, and pull in different directions. Further swelling the page count is the need to intro the story world--and that's a lot of pontificating. My own instructor recognized that with the comment "There is no protagonist." Having made the corrections, I must agree. Without a standout lead, you end up with Battlestar Galactica.
We can try paring back the playlist. For those aghast at the notion, all is not lost. There are ways to keep the prima donnas on stage while letting the top dog stay on top. Consider spinning them off to their own series, or letting them star in sequels.
Word economy makes the most of their time on stage. While we enjoy the minutiae of the hero's journey, there isn't time to indulge just anybody. Here'e what's likely to happen when a secondary character springs a flashback chapter to his childhood: el skippo, amigo--let's get on with it.
Which brings up relevance. If a shadowy thief mixes up a zombie potion, it needs to be sprung on some high-level baddie, sooner rather than later. That justifies the three pages we spent watching him do it. In the Locke Lamora series, two partners use a host of secondaries to con their way into the high life. We don't mind savoring that vicariously, since their knowledge of objects d'art is how they make money to stay in the game.
Still, it pays to concentrate on whomever makes the biggest mark on the story. It's fine to let supporting actors shine if you don't let them get out of control. The stronger they are, the more they help the hero reach his goal, and the more they bring him to life.