You're talking about AoS. I'm not talking about AoS. It's 8th Ed Warhammer Fantasy Battle I'm talking about. In 8th Ed fantasy Skaven had the Strength in Numbers rule (sorry, not safety in numbers, my bad) which gave them +1 to their leadership for each extra rank of 5 or more models after the first rank. Now let us take a look at a horde of 40 Clanrats in 10X4 formation vs a bog-standard unit of 20 in 5X4 formation. At the start of the game, both would receive a lovely +3 bonus to their leadership, giving them a mighty Leadership of 9, same as a unit of Dwarfs or High Elves, because each unit has 3 extra ranks of 5 or more models after the first rank. However, the 5X4 unit would lose a point of Leadership as soon as they suffer their first casualty, because they would by this time have 2 ranks of 5 and a rank of 4 after the first rank, giving them only a +2 bonus rather than +3 and reducing their Ld to 8. The horde, meanwhile, would need to suffer 6 casualties before they would lose that point of Leadership, because by then they would have 2 ranks of 10 and a rank of 4. As such, you would need to kill off a lot more models in the horde to reduce their leadership to their pathetic leadership of 6 so that they would more easily fail leadership tests.
In fact this is the only instance where I allow hordes to be used in my games of Warhammer 8th Ed. I have a 'house rule' when I play fantasy at home that unless you play Skaven you should not use hordes because it is not characteristic of the way the army would fight and because hordes are inflexible.