Glad you're enjoying it & sorry the dates with the feeder girl were a letdown!
Right now, it's not actually possible for the feeders to gain weight at all; they exist solely to fatten the player as much as possible. Resistance chance is based on your affection level.
That's changing very soon, though. I've been working on an update since Christmas to improve the new features I added in the last big release & it's drawing close to being finished now. Many of those improvements are to feeder girls, including retuning resistance chance, a player appetite system that impacts this & the ability to fatten up the feeders. It'll be entirely possible for their efforts to completely backfire & (with enough cardio/weight loss medication) for you to stay slim whilst they are fattened up into extreme obesity!