It isn't the weakest, but the one with the second lowest MORTAL lever. That means the amount/balance of life in the universe in general is screwed. Kais create new life and guide it, Gods of destruction destroy old life so it can be replaced. But with Shin being left as the only surviving Kai at a young age, and Beerus being a lazy fuck, it meant that a lot of things ruined the balance they were suposed to mantain (coughfreezacough). Shin being unable of properly kai, and Beerus destroying planets for stupid shit meant the universe is underpopulated.

Universe 9 had the reverse problem: Due it's God of Destruction being indecise at the time of destroying, the universe grow overpopulated.

Meanwhile, the top 4 universes aren't participation because they're properly balanced. So it's not that their population is stronger, is that their gods are more competent.

He said Universe had the second lowest Mortal Level, which is different from strength.