Isekai Shoukan-sareta ga Kyousei Soukan-sareta Ore ha shikatanaku yaseru koto ni shita.異世界召喚されたが強制送還された俺は仕方なくやせることにした。; I was Summoned to Another World, but I was Forced to Return Home, So I Decided to Lose Some Weight

Haruhiko is a 32 year old fat shut-in. One night he gets hit by a bus and finds himself summoned to a new world. However due to weight limitations the summoning system produces an error and he's sent back home. A princess from the other world accidentally comes back with him. In order to return her to her world he must lose weight.