I’ll do my best to clarify what I can about this series.
While yes, in the first chapter her boyfriend gained some weight, it’s not the focus. It’s mentioned in the second page that he gained 30 kg prior to the comic, and he gains a grand total of 3.4 kg during the chapter itself. The girl, by contrast, gains 15.4 kg, and is shown through the dialogue that she’s way more into it than he is. The boyfriend seems a little self-conscious by comparison, even though he makes a small effort to gain alongside her. On page 1 alone she’s shown sneaking cake during sex (to which her boyfriend explicitly says is something he can’t handle himself), and in the second half of the chapter she’s shown to get all excited with her new weight. She also gets snacks after sex while her boyfriend is resting.
I do think that as someone here said, it’s kinda clear she’s going to turn into the bigger one. Being alongside a fat guy and still looking absolutely huge by comparison is probably what Habutton is going for, compared to his previous works where most of the love interests are skinny lanky men and the girls are usually gaining unintentionally.