http://leagueoflegends.wikia.com/wiki/Kindred
INNATE: Both Lamb and Wolf mark targets to hunt. Scoring a takedown on a marked target permanently empowers Lamb's basic attacks to deal 1.25% of her target's current health bonus on-hit physical damage, stacking with each mark consumed. **The bonus damage is capped at +75 versus monsters. **The mark is removed on the target's death even if Kindred are not involved.
It's already capped.
If they were killing it extremely quickly, then it was likely a combination of Sated Devourer, Blade of the Ruined King, Walk's Frenzy, and their passive doing so.
Well, unless Kindred is bugged. Though I think doing uncapped percentage health damage to Baron or Dragon would have been noticed before they were released.