Isekai Shoukan-sareta ga Kyousei Soukan-sareta Ore wa Shikatanaku Yaseru Koto ni Shita.

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.

(Source: TO Books, translated)