Mainichi Ie ni Kuru Gal ga Kyorikan Zero demo Yasashikunai the Comic

Class queen Manami Fuwa has been gaining weight lately, and to make matters worse, a rival girl is taunting her in class, calling her a "muscle ball with money." When Taichi Utsuki can't contain his thoughts about her weight, he's forced to accompany Fuwa on her diet! A romantic comedy of dieting with a violent, irrational, selfish girl with no sense of emotional distance begins.

(Source: Micro Magazine-sha, translated)