I stumbled upon this one by random luck.
Beware, this is not a “satisfying” or “excellent” exemple but I thought I’d mention it.
“Codename Sailor V” is a small manga that ran in magazines between 1991 and 1997. So be prepared for the qualty of a 90s small manga.
Codename Sailor V basically acts as a prequel to the Sailor Moon manga you might be more familiar with. The ninth chapter of the manga is called “Sailor V versus Devleen” and features weight gain - though due to the chibi and simplistic artstyle, you won’t get any visual satisfaction.
Basically the “Dark Agency” (the supervillains of Sailor Moon) due to their recurent defeats, have lost their energy. One of the members of the Agency, Danburine (aka Devleen) proposes a plan that not only will bring energy to the villains but also money to the Agency. The plan is very simple.
Create a popular chain of chocolate shops that sell very tasty, but also very addictive and very fattening chocolates. They are mostly destined at young girls and women - offering them a lot of free samples. These chocolates are enough to make the girls gain in maybe a week somewhere between four and six kilos. (Note that this is a Valentine’s Day chapter, so Valentine Day is coming, the girls not only will want to try chocolates to find the best for their lovers but also will want to look their best for the day of love).
Then, the second part of the plan is the opening of “Este de Brine” spa, a spa dedicated to helping girls lose weight and get a perfect body - before distributing a lot of flyers around with a “first test free”. As a result, all the girls come to the spa to get “reduced” (Note here something very clever from the part of the villain - when she poses as Este de Brine, the owner of the spa, she actually doesn’t actively force or shame the girls into losing weight. She welcomes them easing their fear saying that heavy girls look better, and chubby girls are more popular with boys - but if they REALLY want to do it, she can help them… Like that no one suspect her real intentions).
And here is the trick. The girls undergo a tiring process… then they look at themselves in the mirror, and find they look thin again, and gorgeous! So they leave the spa… But it is all a trick. The reflection in the mirrors are an illusion spell that will then fade away a few times after they left the spa. The spa doesn’t at all helps the girls lose weight - it rather sucks out their vital energy out of them. And here is the trick… When they go back home, since they are thin again, they want to indulge in their chocolate addiction, which helps them replenish their strength - only for them to become even fatter. So they return there, but this time they will have to pay and they are ready to pay because the effects seem so miraculous… and thus begins an endless cycle.
And here is where us, male-oriented persons, can get some interesting material (though the visuals are very poor). Once Valentine’s Day comes, the girls all give the boys they love chocolates of the fattening brand (Rainbow Chocolates they are called). And the boys adore it - so they begin buying some… which results in them also gaining noticeable weight in a few days - enough for them to gain chubby faces, round bodies, and some even begin waddling (though it is all in chibi form so it is hard to see).
Anyway ultimately Sailor Moon (well actually Sailor Venus at the time, hence the “V” in the title) defeats the villain and breaks her illusion spell and energy-sucking machine… before being puzzled by how none of the chocolate victims have lost weight. Because she believed the weight gain was also some sort of curse - but no, the people really have gained weight and the chocolates were just very fattening. As a result, the only thing the heroes can do is give everyone free diet tips and diet candy…