Saiaku no Avalon

When a hardcore gamer downloads an update that transports him into his favorite game, Dungeon Explorer Chronicles, he expects a life of pretty girls and epic fights. He starts as a student at Adventurers' High, the top school for budding adventurers, eager to dive into the fantastical dungeon that's appeared in modern-day Japan. But one glance in the mirror shatters his dreams of badassery! He's entered the game as Piggy, the love-to-hate overweight villain whose stalker-like obsession with his reluctant fiancée spells his doom. With elitist bullies roaming the school halls and cutthroat adventurers lurking in the dungeon, our hero has to act fast to lose weight and level up if he wants to survive and break free from his scripted fate. Using his peerless knowledge of the game, he's ready to exploit every trick to become the world's greatest unsung hero!

(Source: J-Novel Club)