Nala and Zira had been friends for a long, long time. Ever since they were cubs, they had been sharing meals together, playing together, even finding mates together. Nala consoled Zira when Mufasa had announced Scar as a traitor, his assassination attempt having failed. The two lionesses had, by adulthood, become quite obese- to the enjoyment of their mates. Having an overweight mate was something, in recent times, seen as a good thing, and this was thanks to a new policy that Mufasa had to put in place before he died. Scar was waging a war against the lions, leading the Hyenas with their jaws powerful enough to break a lion's leg in a single bite, in many attacks. Mufasa, although having narrowly survived Scar's assassination attempt, died in battle with Hyenas, acting as a distraction to give his forces a chance to escape. Before he had died, though, Mufasa had put a new policy in place in order to get more recruits for the war: "One shall hunt for their own families feed unless one is to swear service under me or any future ruler as a warrior of the Pride Lands, in which their families shall be given food gathered by the Pride as a whole." Nala, with Simba, was given a great amount of food, being queen of the Pride Lands, and Zira got large amounts of food from the great service her mate was giving. Nala told Simba and Rafiiki of her expansive desires and he gave her 10 gourds filled with potions which allowed her to gain twice as much weight from anything she consumed.
Nala and Zira were kidnapped by poachers, Zira first, then Nala, and they used them to test a serum which was A: cheap to produce, B: quick to produce, and C: was incredibly fattening. They wished to use the serum to cure world hunger, but were worried about side effects, and so here we are. When they gave it to Zira, she immediately began to balloon, layer after layer of fat being added to her frame, her limbs quickly vanishing under her expanding mass. Nala, who had just drank all ten potions at once instead of incrementally like Rafiiki had told her to, exploded, her mass rushing forth in a tidal wave of flabby, flabby flesh.