Okay, yeah, those two scenarios do seem more realistic.
Actually, you could swap Babs and Jess around in that scenario and make JESSICA be the fat girl by the end of it. Like, we know she doesn't eat meat. Maybe, as she's desperately trying to get Babs to eat healthy, she cringes and takes a bite of a hamburger. Her tastebuds explode with delight and she remembers all over again how fucking delicious meat and cheese and dairy are. Cue her stealthily stuffing her face with greasy fast food in between lecturing and training Babs. By the end of the episode Barbara has washboard abs and Jess has turned into a huge-assed fat cow.
For added DC Comics fun, we can throw Larfleeze and the Orange Lantern into the mix and Jess can become consumed with gluttony and greed. We've already had the Red Lanterns and the Star Sapphires show up so I assume the entire emotional spectrum is canon to this show.
And the episode ends with a very fat Jess being drilled into jogging by Babs, in an ironic reversal of their roles at the start of the episode.