*Flashback*
"Ssh, I thought I heard something." Indeed, that night the Joker had heard a strange noise, even though he still took advantage of the distraction to push Harley into the vat. The noise that night was in fact Batgirl. She had been patrolling the warehouse district of Gotham, when she spotted someone sneaking into Ajax Chemicals. She only witnessed Harley entering after words though, so she didn't think to contact Batman.
She grappled up onto the roof, and mad her way past the sky light. As she lowered herself down though, she almost lost her grip and lightly yelped. Lucky for her the Joker had other occupations.
What happened next, she could have never predicted. As she was about to leap away from the vat beneath her, she saw in the distance, Harley pull the Joker into the vat. As she hung there, stunned by the struggle she heard, she soon witnessed Harley rising out of the chemicals, now with a human sized belly.

"Oh my god, oh my-AAAAAAAAAHHH!!! *SPLASH* " taken completely aback by what she had seen, Barbarra lost her grip and she to fell into a tub of Chemical V. After resurfacing however, she did not go into a ravenous state...yet.
After she got to the ladder though, like Harley she fell out once she got over the edge. There she layed on the ground, coughing up some of that glop she just swam in. Dazed and confused, Batgirl's natural instinct was to get back to the Batcave.
As she stumbled out the door, Harley was a few yards away moaning "Oooh, Mr. J.".
*End Flashback*

"Well, at least now you don't look anorexic." Harley insulted. Batgirl glared at her fat foe, and said "Yeah well even when I was skinny I still kicked your fat butt.". Harley then turned around, and slapped her back side "Well come and get it, Fat Girl.".
Suddenly, the windows shattered, as two other figures swung in. Batman, and Robin the boy wonder!
"Woah, and I thought Batgirl was big." Robin stared at the huge villainess. Batman wasn't too surprised after seeing Barbarra.
"Get 'im!" Harley shouted. The henchman charged.

Batman tossed some batarangs, hitting two guys in the face. A few others came after Robin meanwhile, but he used his grapple zip up, and then toss some pellets down. They released tear gas.
A few others tried to get their guns, but Batgirl ran them over with her bulk. Soon afterwords, she came after Harley, but Poison Ivy stepped in the way "Back off.".
She tossed some seeds, and they grew into vines on contact with her body. Batgirl struggled, but the plant was growing to fast. It even started to tear her already stretched to the brim costume.
Just when it seemed like she'd be constricted though, she did something completely unexpected.

"No!" Ivy watched in horror as her baby's vines were chomped by Fat Girl. Chomp after chomp, bite after bite, soon only scraps of the plant were on the floor. " *Gulp* I have been needing more greens in my diet." she swallowed. Ivy could see that the enemy's power equaled Harley's, and as much as she would love to see two fat women wrestle with each other, her logical mind blurted out "Harley, we need a smokescreen!!".
"Smokescreen?...oh I got ya." Harley quickly turned her back to her opponents, bent over slightly, and let it rip.

*PPPPPPPPPPPPPPPPPPPPPPPPPPPRRRRRRRRRRRRRRRRRRRRRRRRRRRRRUUUUUUUUP*

Everyone choked on the green-ish gaseous cloud. During which though, Harley rounded up her henchman, while Ivy got in the driver seat of a parade float. After they got in, and established that it could support a lot of weight, Harley pushed the candy clown manicine out of the way, and they were off.
Eventually the air cleared. Batman and Robin both had gas masks on by now, but Batgirl did not. Not just because she couldn't reach her utility belt, but the gas didn't bother her as intensely as it had the others.

"They're gone, but they won't stay hidden for long." Batman knealed at the tire tracks. "I don't think Harley will be too hard to find, now I see how she ate the Joker." Robin remarked. "Yeah, but since we lost them...." Barbarra got her answer when Batman said "Don't worry Barbarra, until they show up again I will be working on synthesizing a cure.". He went over to one of the vats to get a sample.
Barbarra was happy to hear that. Unlike Harley, she was not happy with her new body. She was way too big to sneak around in the shadows. Her attacks had also been slowed, but luckily she could ram her enemies now. Not to mention all the food she had to eat yesterday to get this big.
Her costume was the biggest problem. Her mask alone felt tighter because of her puffy cheeks. Her belly was exposed only in the lower region, but enough to show her belly button. Her legs were fortunately big enough to support her weight, but they still stretched the fabric. Her huge hooters had an even more dire effect on the top half. The only thing that matched, and actually surpassed it, was her rear.
Speaking of which, when no one looked, Barbarra dug at her booty. The tightly stretched fabirc was also giving her a wedgie. There wasn't even any underwear to block it anymore.

Finally, the trio began to walk towards the huge backdoor. Batman and Robin were out long before Barbarra was.
She stopped suddenly, looking below her, she spotted a henchman Harley had left behind. He was out cold, most likely from her fart. Barbarra looked back to see that her partners were out of sight. She looked back at the man, and licked her lips.
"Wait! What am I thinking!!?". she thought.
How could she have thought, even for a moment, about eating a person. She could never kill anyone, she was the daughter of the police commissioner. Plus, this guy would mean more unwanted fat.

Her belly roared "Oh, but I'm so hungry!".